.    GIFT   OF 

MICHAEL 'REESE 


THE  FILTRATION 


OF 


PUBLIC  WATER-SUPPLIES 


BY 

ALLEN    HAZEN, 

LATE  CHEMIST  IN   CHARGE  OF  THE   LAWRENCE  EXPERIMENT  STATION   OF  THE  MASSACHUSETTS 

STATE  BOARD  OF  HEALTH,  AND  CHEMIST  OF  THE  DEPARTMENT  OF  WATER-SUPPLY 

AND  SEWERAGE   OF  THE  WORLD'S   COLUMBIAN   EXPOSITION  }     MEMBER   OF 

THE  BOSTON  SOCIETY  OF  CIVIL   ENGINEERS,  THE  NEW  ENGLAND 

WATER-WORKS  ASSOCIATION,   THE  AMERICAN  PUBLIC 

HEALTH  ASSOCIATION,  ETC. 


SECOND    EDITION. 
FIRST    THOUSAND. 


NEW  YORK : 

JOHN    WILEY    &    SONS. 

LONDON:    CHAPMAN    &    HALL,    LIMITED. 
1896. 


Copyright,  1895, 

BY 
ALLEN  HAZEN. 


ROBERT  DRUMMOND,  KLKCTROTYPER  AND  PRINTER,   NEW  YORK. 


PREFACE. 


THE  subject  of  water-filtration  is  commencing  to  receive  a 
great  deal  of  attention  in  the  United  States.  The  more  densely 
populated  European  countries  were  forced  to  adopt  filtration 
many  years  ago,  to  prevent  the  evils  arising  from  the  unavoidable 
contaminations  of  the  rivers  and  lakes  which  were  the  only  avail- 
able sources  for  their  public  water-supplies;  and  it  has  been 
found  to  answer  its  purpose  so  well  that  at  the  present  time  cities 
in  Europe  nearly  if  not  quite  equal  in  population  to  all  the  cities 
of  the  United  States  are  supplied  with  filtered  water. 

Many  years  ago,  when  the  whole  subject  of  water-supply  was 
still  comparatively  new  in  this  country,  filtration  was  considered  as 
a  means  for  rendering  the  waters  of  our  rivers  suitable  for  the  pur- 
pose of  domestic  water-supply.  St.  Louis  investigated  this  subject 
in  1866,  and  the  engineer  of  the  St.  Louis  Water  Board,  the  late 
Mr.  J.  P.  Kirkwood,  made  an  investigation  and  report  upon  Euro- 
pean methods  of  filtration  which  was  published  in  1869,  and  was 
such  a  model  of  full  and  accurate  statement  combined  with 
clearly-drawn  conclusions  that,  up  to  the  present  time,  it  has  re- 
mained the  only  treatise  upon  the  subject  in  English,  notwith- 
standing the  great  advances  which  have  been  made,  particularly 
in  the  last  ten  years,  with  the  aid  of  knowledge  of  the  bacteria 
and  the  germs  of  certain  diseases  in  water. 

Unfortunately  the  interest  in  the  subject  was  not  maintained 
in  America,  but  was  allowed  to  lag  for  many  years ;  it  was 
cheaper  to  use  the  water  in  its  raw  state  than  it  was  to  purify  it ; 
the  people  became  indifferent  to  the  danger  of  such  use,  and 


IV  PREFACE. 

the  disastrous  epidemics  of  cholera  and  typhoid  fever,  as  well  as 
of  minor  diseases,  which  so  often  resulted  from  the  use  of  pol- 
luted water,  were  attributed  to  other  causes.  With  increasing: 
study  and  diffusion  of  knowledge  the  relations  of  water  and  dis- 
ease are  becoming  better  known,  and  the  present  state  of  things 
will  not  be  allowed  to  continue;  indeed  at  present  there  is  in- 
quiry at  every  hand  as  to  the  methods  of  improving  waters. 

The  one  unfortunate  feature  is  the  question  of  cost.  Not 
that  the  cost  of  filtration  is  excessive  or  beyond  the  means  of 
American  communities  ;  in  point  of  fact,  exactly  the  reverse  is 
the  case  ;  but  we  have  been  so  long  accustomed  to  obtain  drink- 
ing-water without  expense  other  than  pumping  that  any  cost 
tending  to  improved  quality  seems  excessive,  thus  affording  a 
chance  for  the  installation  of  inferior  filters,  which  by  failing  to 
produce  the  promised  results  tend  to  bring  the  whole  process 
into  disrepute,  since  few  people  can  distinguish  between  an  ade- 
quate filtration  and  a  poor  substitute  for  it.  It  is  undoubtedly 
true  that  improvements  are  made,  and  will  continue  to  be  made, 
in  processes  of  filtration ;  so  it  will  often  be  possible  to  reduce  the 
expense  of  the  process  without  decreasing  the  efficiency,  but 
great  care  must  be  exercised  in  such  cases  to  maintain  the  con- 
ditions really  essential  to  success. 

In  the  present  volume  I  have  endeavored  to  explain  briefly 
the  nature  of  filtration  and  the  conditions  which,  in  half  a  cen- 
tury of  European  practice,  have  been  found  essential  for  success- 
ful practice,  with  a  view  of  stimulating  interest  in  the  subject, 
and  of  preventing  the  unfortunate  and  disappointing  results 
which  so  easily  result  from  the  construction  of  inferior  filters. 
The  economies  which  may  possibly  result  by  the  use  of  an  infe- 
rior filtration  are  comparatively  small,  and  it  is  believed  that  in 
those  American  cities  where  filtration  is  necessary  or  desirable  it 
will  be  found  best  in  every  case  to  furnish  filters  of  the  best 
construction,  fully  able  to  do  what  is  required  of  them  with  ease 
and  certainty. 


CONTENTS. 


PAGE- 
CHAPTER  I.  INTRODUCTION        .   v  .....        ,       .       .        .       .      i 
II.  CONTINUOUS  FILTERS  AND  THEIR  CONSTRUCTION     .       .      5 
Sedimentation-basins     .         .        .;..'.        .         .        .8 
Size  of  Filter-beds          .        .         *        .        .         .     .   .     10 
Covers  for  Filters    .        .        ...        .        •         •        .12 

III.  FILTERING-MATERIALS    .       .       .       .'     ...  ;.-V.-     .        .    19- 

Sand        *        .        .        . I9< 

Gravel     .        .         .         .        .         .  .         .        .31 

Underdrains    .        .        «        .        .-               .         .         •     35» 
Depth  of  Water  on  Filters 41 

IV.  RATE  OF  FILTRATION  AND  Loss  OF  HEAD         .        .        .43 

Rate  of  Filtration  . 43 

Loss  of  Head  and  Apparatus  for  regulating  it  .48 

Limit  to  the  Loss  of  Head     .        .        .        .  '  .,    .        .56 

V.  CLEANING  FILTERS        ...       ...       .  '     ,  .       .64 

Scraping  .  .  '"* 64 

Frequency  of  Scraping 68 

Sand-washing  ....-••  .  .  .  .  .  72; 

VI.  THEORY  AND  EFFICIENCY  OF  FILTRATION  .  .  .  -79* 

Bacterial  Examination  of  Waters 891 

VII.  INTERMITTENT  FILTRATION 93; 

The  Lawrence  Filter       .        .  .        .         .        .96) 

The  Chemnitz  Filter      .         .         .         .         ...         .  ioo> 

VIII.  OTHER  METHODS  OF  FILTRATION         .        .        .        .        .  106- 

Mechanical  Filters  without  Coagulents  .  .  .  ro6 

The  Use  of  Alum  .  .  .  ' 109.' 

Precipitated  Alumina  and  other  Chemicals  .  .  .  113, 

The  Use  of  Metallic  Iron 114. 

Household  Filters 115; 

IX.  COST  AND  ADVANTAGES  OF  FILTRATION  .  .  .  .118 

Cost  of  Filtration  . ii& 

Objects  of  Filtration       .        .         .         •         .         •         .122- 

What  Waters  require  Filtration?  .         .        •        •         .130 

X.  CONCLUSIONS  .       .       .       ...       ...       .  133 

Water  and  Disease  .  ...  s  .  •  .  133- 

T 


VI  CONTENTS. 

PAGE 

APPENDIX  I.  GERMAN  OFFICIAL  REGULATION  IN  REGARD  TO  FILTRATION  139 
II.    EXTRACTS    FROM    DR.    REINCKE'S  REPORT   UPON   THE 

HEALTH  OF  HAMBURG  FOR  1892 144 

III.  METHODS  OF  SAND-ANALYSIS 151 

IV.  STATISTICS  OF  SOME  FILTERS       .       .        .       ...       .159 

V.  WATER-SUPPLY  OF  LONDON         .        .        .        .  .161 

VI.  WATER-SUPPLY  OF  BERLIN 167 

VII.  WATER-SUPPLY  OF  ALTONA 171 

VIII.  WATER-SUPPLY  OF  HAMBURG 175 

IX.  NOTES  ON  SOME  OTHER  EUROPEAN  SUPPLIES      .       .       .178 

X.  LITERATURE  OF  FILTRATION 183 

XI.  PLANS  OF  PROPOSED  FILTERS  FOR  BOSTON  METROPOLI- 
TAN WATER  DISTRICT 190^ 

INDEX  191 


UNITS   EMPLOYED. 


The  units  used  in  this  work  are  uniformly  those  in  common  use 
in  America,  with  the  single  exception  of  data  in  regard  to  sand-grain 
sizes,  which  are  given  in  millimeters.  The  American  units  were  not 
selected  because  the  author  prefers  them  or  considers  them  partic- 
ularly well  suited  to  filtration,  but  because  he  feared  that  the  use 
of  the  more  convenient  metric  units  in  which  the  very  comprehen- 
sive records  of  Continental  filter  plants  are  kept  would  add  to  the 
difficulty  of  a  clear  comprehension  of  the  subject  by  those  not 
familiar  with  those  units,  and  so  in  a  measure  defeat  the  object  of 
the  book. 


TABLE  OF  EQUIVALENTS. 

Unit.  Metric  Equivalent. 

Foot  ................         0.3048  meter 

Mile  .........  .  .......  1609.34      meters 

Acre  ................  4047          square  meters 

Gallon*  ..............         3.785    liters 

i  million  gallons  .....  3785  cubic  meters 

Cubic  yard  .  ..........        0.7645  cubic  meters 


I  million  gallons  per  ) 
acre  daily  j 


meter  in  dePth  I 
of  water  daily    J 


Reciprocal. 
3.2808 

0.0006214 

0.0002471 

0.26417 

0.00026417 

1.308 

T  070 


*The  American  gallon  is  231  cubic  inches  or  0.8333  °f  tne  imperial  gallon.     In 
this  work  American  gallons  are  always  used,  and  English  quantities  are  stated  in 

American,  not  imperial,  gallons. 

vii 


ACKNOWLEDGMENT. 


I  WISH  to  acknowledge  my  deep  obligation  to  the  large  number 
of  European  engineers,  directors,  and  superintendents  of  water- 
works, and  to  the  health  officers,  chemists,  bacteriologists,  and  other 
officials  who  have  kindly  aided  me  in  studying  the  filtration-works 
in  their  respective  cities,  and  who  have  repeatedly  furnished  me  with 
valuable  information,  statistics,  plans,  and  reports. 

To  mention  all  of  them  would  be  impossible,  but  I  wish  particu- 
larly to  mention  Major-General  Scott,  Water-examiner  of  London ; 
Mr.  Mansergh,  Member  of  the  Royal  Commission  on  the  Water- 
supply  of  the  Metropolis ;  Mr.  Bryan,  Engineer  of  the  East  London 
Water  Company  ;  and  Mr.  Wilson,  Manager  of  the  Middlesborough 
Water-works,  who  have  favored  me  with  much  valuable  information. 

In  Holland  and  Belgium  I  am  under  special  obligations  to 
Messrs.  Van  Hasselt  and  Kemna,  Directors  of  the  water  companies 
at  Amsterdam  and  Antwerp  respectively ;  to  Director  Stang  of  the 
Hague  Water-works  ;  to  Dr.  Van't  Hoff,  Superintendent  of  the 
Rotterdam  filters  ;  and  to  my  friend  H.  P.  N.  Halbertsma,  who,  as 
consulting  engineer,  has  built  many  of  the  Dutch  water-works. 

In  Germany  I  must  mention  Profs.  Friihling,  at  Dresden,  and 
Fliigge,  at  Breslau  ;  Andreas  Meyer,  City  Engineer  of  Hamburg; 
and  the  Directors  of  water-works,  Beer  at  Berlin,  Dieckmann  at 
Magdeburg,  Nau  at  Chemnitz,  and  Jockmann  at  Liegnitz,  as  well  as 
the  Superintendent  Engineers  Schroeder  at  Hamburg,  Debusmann 
at  Breslau,  and  Anklamm  and  Piefke  at  Berlin,  the  latter  the  dis- 
tinguished head  of  the  Stralau  works,  the  first  and  most  widely 
known  upon  the  Continent  of  Europe. 

I  have  to  acknowledge  my  obligation  to  City  Engineer  Sechner 
at  Budapest,  and  to  the  Assistant  Engineer  in  charge  of  water-works, 
Kajlinger;  to  City  Engineer  Peters  and  City  Chemist  Bertschinger 


X  A  CKNO  WLED  GMENT. 

at  Zurich ;  and  to  Assistant  Engineer  Regnard  of  the  Compagnie 
Gen£rale  des  Eaux  at  Paris. 

On  this  side  of  the  Atlantic  also  I  am  indebted  to  Hiram  F. 
Mills,  C.E.,  under  whose  direction  I  had  the  privilege  of  conducting 
for  nearly  five  years  the  Lawrence  experiments  on  filtration  ;  to 
Profs.  Sedgwick  and  Drown  for  the  numerous  suggestions  and 
friendly  criticisms,  and  to  the  latter  for  kindly  reading  the  proof  of 
this  volume ;  to  Mr.  G.  W.  Fuller  for  full  information  in  regard 
to  the  more  recent  Lawrence  results ;  to  Mr.  H.  W.  Clark  for 
the  laborious  examination  of  the  large  number  of  samples  of  sands 
used  in  actual  filters  and  mentioned  in  this  volume;  and  to  Mr. 
Desmond  FitzGerald  for  unpublished  information  in  regard  to  the 
results  of  his  valuable  experiments  on  filtration  at  the  Chestnut  Hill 
Reservoir,  Boston. 

ALLEN  HAZEN. 

BOSTON,  April,  1895. 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


CHAPTER  I. 
INTRODUCTION. 

THE  rapid  and  enormous  development  and  extension  of 
water-works  in  every  civilized  country  during  the  past  forty 
years  is  a  matter  which  deserves  our  most  careful  consideration,, 
as  there  is  hardly  a  subject  which  more  directly  affects  the  health 
and  happiness  of  almost  every  single  inhabitant  of  all  cities  and 
large  towns. 

Considering  the  modern  methods  of  communication,  and  the 
free  exchange  of  ideas  between  nations,  it  is  really  marvellous 
how  each  country  has  met  its  problems  of  water-supply  from  its 
own  resources,  and  often  without  much  regard  to  the  methods 
which  had  been  found  most  useful  elsewhere.  England  has 
secured  a  whole  series  of  magnificent  supplies  by  impounding 
the  waters  of  small  streams  in  reservoirs  holding  enough  water 
to  last  through  dry  periods,  while  on  Continental  Europe  such 
supplies  are  hardly  known.  Germany  has  spent  millions  upon 
millions  in  purifying  turbid  and  polluted  river-waters,  while 
France  and  Austria  have  striven  for  mountain-spring  waters  and 
have  built  hundreds  of  miles  of  costly  aqueducts  to  secure  them. 
In  the  United  States  an  abundant  supply  of  some  liquid  has  too- 
often  been  the  objective  point,  and  the  efforts  have  been  most 


2  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

successful,  the  American  works  being  entirely  unrivalled  in  the 
volumes  of  their  supplies.  I  do  not  wish  to  imply  that  quality 
has  been  entirely  neglected  in  our  country,  for  many  cities  and 
towns  have  seriously  and  successfully  studied  their  problems, 
with  the  result  that  there  are  hundreds  of  water-supplies  in  the 
United  States  which  will  compare  favorably  upon  any  basis  with 
supplies  in  any  part  of  the  world ;  but  on  the  other  hand  it  is 
equally  true  that  there  are  hundreds  of  other  cities,  including 
some  among  the  largest  in  the  country,  which  supply  their 
citizens  with  turbid  and  unhealthy  waters  which  cannot  be 
regarded  as  anything  else  than  a  national  disgrace  and  a  menace 
to  our  prosperity. 

One  can  travel  through  England,  Belgium,  Holland,  Ger- 
many, and  large  portions  of  other  European  countries  and  drink 
the  water  at  every  city  visited  without  anxiety  as  to  its  effect 
upon  his  health.  It  has  not  always  been  so.  Formerly  Euro- 
pean capitals  drank  water  no  better  than  that  so  often  dis- 
pensed now  in  America.  It  is  but  two  years  since  Germany's 
great  commercial  centre,  Hamburg,  having  a  water-supply 
essentially  like  those  of  Philadelphia,  Pittsburg,  Cincinnati, 
St.  Louis,  New  Orleans,  and  a  hundred  other  American  cities, 
paid  a  penalty  in  one  month  of  eight  thousand  lives  for  its  care- 
lessness. The  lesson  was  a  dear  one,  but  it  was  not  wasted. 
Hamburg  now  has  a  new  and  wholesome  supply,  and  other  Ger- 
man cities  the  qualities  of  whose  waters  were  open  to  question 
have  been  forced  to  take  active  measures  to  better  their  condi- 
tions. We  also  can  learn  something  from  their  experience. 

There  are  three  principal  methods  of  securing  a  good  water- 
supply  for  a  large  city.  The  first  consists  of  damming  a  stream 
from  an  uninhabited  or  but  sparsely  inhabited  watershed,  thus 
forming  an  impounding  reservoir.  This  method  is  extensively 
used  in  England  and  in  the  United  States.  In  the  latter  most  of 
the  really  good  and  large  supplies  are  so  obtained.  It  is  only 
applicable  to  places  having  suitable  watersheds  within  a  reason- 


IN7^RODUCTION.  3 

able  distance,  and  there  are  large  regions  where,  owing  to  geo- 
logical and  other  conditions,  it  cannot  be  applied.  It  is  most 
useful  in  hilly  and  poor  farming  countries,  as  in  parts  of  England 
and  Wales,  in  the  Atlantic  States,  and  in  California.  It  cannot 
be  used  to  any  considerable  extent  in  level  and  fertile  countries 
which  are  sure  to  be  or  to  become  densely  populated,  as  is  the 
case  with  large  parts  of  France  and  Germany  and  in  the  Middle 
States. 

The  second  method  is  to  secure  ground-water,  that  is,  spring 
or  well  water,  which  by  its  passage  through  the  ground  has- 
become  thoroughly  purified  from  any  impurities  which  it  may 
have  contained.  This  was  the  earliest  and  is  the  most  widely 
used  method  of  securing  good  water.  It  is  specially  adapted 
to  small  supplies.  Under  favorable  geological  conditions  very 
large  supplies  have  been  obtained  in  this  manner.  In  Europe 
Paris,  Vienna,  Budapest,  Munich,  Cologne,  Leipzig,  Dresden^ 
a  part  of  London,  and  very  many  smaller  places  are  so  supplied. 
This  method  is  also  extensively  used  in  the  United  States  for 
small  and  medium-sized  places,  and  deserves  to  be  most  care- 
fully studied,  and  used  whenever  possible,  but  is  unfortunately 
limited  by  geological  conditions  and  cannot  be  used  except  in 
a  fraction  of  the  cases  where  supplies  are  required.  No  ground- 
water  supplies  yet  developed  in  the  United  States  are  comparable 
in  size  to  those  used  in  Europe. 

The  third  process  of  securing  a  good  water-supply  is  by  means 
of  filtration  of  surface  waters  which  would  otherwise  be  unsuit- 
able for  domestic  purposes.  The  methods  of  filtration,  which  it 
is  the  purpose  of  this  volume  to  explain,  are  beyond  the  experi 
mental  stage ;  they  are  now  applied  to  the  purification  of  the 
water-supplies  of  European  cities  with  an  aggregate  population 
of  at  least  20,000,000  people.  In  the  United  States  there  are 
possibly  one  two  hundredth  as  many  people  so  supplied,  and  in 
most  of  these  cases  the  filters  are  of  quite  recent  introduction. 
As  far  as  the  bulk  of  our  people,  even  those  in  official  positions, 


4  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

are  concerned,  it  is  no  exaggeration  to  say  that  the  modern 
methods  of  filtration  are  unknown  on  this  side  of  the  Atlantic. 
In  Europe  filtration  has  been  practised  with  continually  improv- 
ing methods  since  1839,  an<3  the  process  has  steadily  received 
wider  and  wider  application.  It  has  been  most  searchingly 
investigated  in  its  hygienic  relations,  and  has  been  repeatedly 
found  to  be  a  most  valuable  aid  in  reducing  mortality.  The 
conditions  under  which  satisfactory  results  can  be  obtained  are 
now  tolerably  well  known,  so  that  filters  can  be  built  in  the 
United  States  with  the  utmost  confidence  that  the  result  will  not 
be  disappointing. 

The  cost  of  filtration,  although  considerable,  is  not  so  great 
as  to  put  it  beyond  the  reach  of  American  cities.  It  may  be 
roughly  estimated  that  the  cost  of  filtration,  with  all  necessary 
interest  and  sinking  funds,  will  add  10  per  cent  to  the  average 
cost  of  water  as  at  present  supplied. 

It  may  be  confidently  expected  that  when  the  facts  are  better 
understood  and  realized  by  the  American  public,  we  shall 
abandon  the  present  filthy  and  unhealthy  habit  of  drinking 
polluted  river  and  lake  waters,  and  shall  put  the  quality  as  well 
as  the  quantity  of  our  supplies  upon  a  level  not  exceeded  by 
those  of  any  country. 


CONTINUOUS  FILTERS  AND    THEIR   CONSTRUCTION. 


CHAPTER  II. 
CONTINUOUS  FILTERS  AND  THEIR  CONSTRUCTION. 

FILTRATION  of  water  consists  in  passing  it  through  some 
substance  which  retains  or  removes  some  of  its  impurities.  In 
its  simplest  form  filtration  is  a  straining  process,  and  the  results 
obtained  depend  upon  the  fineness  of  the  strainer,  and  this  in 
turn  is  regulated  by  the  character  of  the  water  and  the  uses  to 
which  it  is  to  be  put.  Thus  in  the  manufacture  of  paper  an 
enormous  volume  of  water  is  required  free  from  particles  which, 
if  they  should  become  imbedded  in  the  paper,  would  injure  its 
appearance  or  texture.  Obviously  for  this  purpose  the  removal 
of  the  smaller  particles  separately  invisible  to  the  unaided  eye,  and 
thus  not  affecting  the  appearance  of  the  paper,  and  the  removal 
of  which  would  require  the  use  of  a  finer  filter  at  increased 
expense,  would  be  a  simple  waste  of  money.  When,  however, 
a  water  is  to  be  used  for  a  domestic  water  supply  and  transpar- 
ency is  an  object,  the  still  finer  particles  which  would  not  show 
themselves  in  paper,  but  which  are  still  able,  in  bulk,  to  render  a 
water  turbid,  should  be  as  far  as  possible  removed,  thus  necessi- 
tating a  finer  filter ;  and,  when  there  is  reason  to  think  that  the 
water  contains  the  germs  of  disease,  the  filter  must  be  fine 
enough  to  remove  with  certainty  those  organisms  so  extraordi- 
narily small  that  millions  of  them  may  exist  in  a  glass  of  water 
without  imparting  a  visible  turbidity. 

It  is  now  something  over  half  a  century  since  the  first  suc- 
cessful attempts  were  made  to  filter  public  water-supplies,  and 
there  are  now  hundreds  of  cities  supplied  with  clear,  healthy, 
filtered  water.  (Appendix  IV.)  While  the  details  of  the  filters 


6  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

used  in  different  places  present  considerable  variations,  the  general 
form  is,  in  Europe  at  least,  everywhere  the  same.  The  most  im- 
portant parts  of  a  filter  are  shown  by  the  accompanying  sketch, 


FIG.  i. — SKETCH  SHOWING  GENERAL  ARRANGEMENT  OF  FILTER  PLANTS. 

in  which  the  dimensions  are  much  exaggerated.  The  raw  water 
is  taken  from  the  river  into  a  settling-basin,  where  the  heaviest 
mud  is  allowed  to  settle.  In  the  case  of  lake  and  pond  waters 
the  settling-tank  is  dispensed  with,  but  it  is  essential  for  turbid 
river-water,  as  otherwise  the  mud  clogs  the  filter  too  rapidly. 
The  partially  clarified  water  then  passes  to  the  filter,  which 
consists  of  a  horizontal  layer  of  rather  fine  sand  supported  by 
gravel  and  underdrained,  the  whole  being  enclosed  in  a  suita- 
ble basin  or  tank.  The  water  in  passing  through  the  sand  leaves 
behind  upon  the  sand  grains  the  extremely  small  particles  which 
were  too  fine  to  settle  out  in  the  settling-basin,  and  is  quite  clear 
as  it  goes  from  the  gravel  to  the  drains  and  the  pumps,  which 
forward  it  to  the  reservoir  or  city. 

The  passages  between  the  grains  of  sand  through  which  the 
water  must  pass  are  extremely  small.  If  the  sand  grains  were 
spherical  and  -^  of  an  inch  in  diameter,  the  openings  would  only 
allow  the  passage  of  other  spheres  -g^  °f  an  incn  in  diameter, 
and  with  actual  irregular  sands  much  finer  particles  are  held 
back.  As  a  result  the  coarser  matters  in  the  water  are  re- 
tained on  the  surface  of  the  sand,  where  they  quickly  form  a 
layer  of  sediment,  which  itself  becomes  a  filter  much  finer  than 
the  sand  alone,  and  which  is  capable  of  holding  back  under  suit- 
able conditions  even  the  bacteria  of  the  passing  water.  The 
water  which  passes  before  this  takes  place  may  be  less  perfectly 


CONTINUOUS  FILTERS  AND    THEIR   CONSTRUCTION.  7 

filtered,  but  even  then,  the  filter  may  be  so  operated  that  nearly 
all  of  the  bacteria  will  be  deposited  in  the  sand  and  not  allowed 
to  pass  through  into  the  effluent. 

As  the  sediment  layer  increases  in  thickness  with  continued 
filtration,  increased  pressure  is  required  to  drive  the  desired 
volume  of  water  through  its  pores,  which  are  ever  becoming 
smaller  and  reduced  in  number.  When  the  required  quantity 
of  water  will  no  longer  pass  with  the  maximum  pressure  allowed, 
it  is  necessary  to  remove,  by  scraping,  the  sediment  layer,  which 
should  not  be  more  than  an  inch  deep.  This  layer  contains  most 
of  the  sediment,  and  the  remaining  sand  will  then  act  almost  as 
new  sand  would  do.  The  sand  removed  may  be  washed  for  use 
again,  and  eventually  replaced  when  the  sand  layer  becomes  too 
thin  by  repeated  scrapings.  These  operations  require  that  the 
filter  shall  be  temporarily  out  of  use,  and  as  water  must  in 
general  be  supplied  without  intermission,  a  number  of  filters  are 
built  together,  so  that  any  of  them  can  be  shut  out  without 
interfering  with  the  action  of  the  others. 

The  arrangement  of  filters  in  relation  to  the  pumps  varies 
with  local  conditions.  With  gravity  supplies  the  filters  are 
usually  located  below  the  storage  reservoir,  and,  properly  placed, 
involve  only  a  few  feet  loss  of  head. 

In  the  case  of  tidal  rivers,  as  at  Antwerp  and  Rotterdam, 
the  quality  of  the  raw  water  varies  with  the  tide,  and  there  is  a 
great  advantage  in  having  the  settling-basins  low  enough  so 
that  a  whole  day's  supply  can  be  rapidly  let  in  when  the 
water  is  at  its  best,  without  pumping.  At  Antwerp  the  filters 
are  higher,  and  the  water  is  pumped  from  the  settling  basins  to 
them,  and  again  from  the  reservoir  receiving  the  effluents  from 
the  filters  to  the  city.  In  several  of  the  London  works  (East 
London,  Grand  Junction,  Southwark  and  Vauxhall,  etc.)  the 
settling-basins  are  lower  than  the  river,  and  the  filters  are  still 
lower,  so  that  a  single  pumping  suffices,  that  coming  between 
the  filter  and  the  city,  or  elevated  distributing  reservoir. 


FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

In  many  other  English  filters  and  in  most  German  works  the 
settling-basins  and  filters  are  placed  together  a  little  higher  than 
the  river,  thus  avoiding  at  once  trouble  from  floods  and  cost 
for  excavation.  The  water  requires  to  be  pumped  twice,  once 
before  and  once  after  filtration.  At  Altona  the  settling-basins  and 
filters  are  placed  upon  a  hill,  to  which  the  raw  Elbe  water  is 
pumped,  and  from  which  it  is  supplied  to  the  city  after  filtration 
by  gravity  without  further  pumping.  The  location  of  the  works 
in  this  case  is  said  to  have  been  determined  by  the  location  of 
a  bed  of  sand  suitable  for  filtration  on  the  spot  where  the  filters 
were  built. 

When  two  pumpings  are  required  they  are  frequently  done, 
especially  in  the  smaller  places,  in  the  same  pumping-station, 
with  but  one  set  of  boilers  and  engines,  the  two  pumps  being 
connected  to  the  same  engine.  The  cost  is  said  to  be  only 
slightly  greater  than  that  of  a  single  lift  of  the  same  total  height. 
In  very  large  works,  as  at  Berlin  and  Hamburg  and  some  of 
the  London  companies,  two  separate  sets  of  pumping  machinery 
involve  less  extra  cost  relatively  than  would  be  the  case  with 
smaller  works. 

SEDIMENTATION-BASINS. 

Kirkwood*  found  in  1866  that  sedimentation-basins  were  es- 
sential to  the  successful  treatment  of  turbid  river-waters,  and 
subsequent  experience  has  not  in  any  way  shaken  his  conclusion. 
The  German  works  visited  by  him,  Berlin  (Stralau)  and  Altona, 
were  both  built  by  English  engineers,  and  their  settling-basins 
did  not  differ  materially  from  those  of  corresponding  works  in 
England.  Since  that  time,  however,  there  has  been  a  well- 
marked  tendency  on  the  part  of  the  German  engineers  to  use 
smaller,  while  the  English  engineers  have  used  much  larger  sedi- 
mentation-basins, so  that  the  practices  of  the  two  countries  are 

*  Filtration  of  River  Waters.     Van  Nostrand  &  Co.,  1869. 


CONTINUOUS  FILTERS  AND    THEIR   CONSTRUCTION.  9 

now  widely  separated,  the  difference  no  doubt  being  in  part  at 
least  due  to  local  causes. 

Kirkwood  found  sedimentation-basins  at  Altona  with  a  capac- 
ity of  2\  times  the  daily  supply.  In  1894  the  same  basins 
were  in  use,  although  the  filtering  area  had  been  increased 
from  0.82  acre  to  2.20  acres,  and  still  more  filters  were  in  course 
of  construction,  and  the  average  daily  quantity  of  water  had  in- 
creased from  600,000  to  4,150,000  gallons  in  1891-2,  or  more  than 
three  times  the  capacity  of  the  sedimentation-basins.  In  1890 
the  depth  of  mud  deposited  in  these  basins  was  reported  to  be 
two  feet  deep  in  three  months.  At  Stralau  in  Berlin,  also,  in 
the  same  time  the  filtering  area  was  nearly  doubled  without  in- 
creasing the  size  of  the  sedimentation-basins,  but  the  Spree  at 
this  point  has  such  a  slow  current  that  it  forms  itself  a  natural 
sedimentation-basin.  At  Magdeburg  on  the  Elbe  works  were 
built  in  1876  with  a  filtering  area  of  1.92  acres,  and  a  sedimen- 
tation-basin capacity  of  11,300,000  gallons,  but  in  1894  half  of  the 
latter  had  been  built  over  into  filters,  which  with  two  other  filters 
gave  a  total  filtering  surface  of  3.90  acres,  with  a  sedimentation- 
basin  capacity  of  only  5,650,000  gallons.  The  daily  quantity  of 
water  pumped  for  1891-2  was  5,000,000  gallons,  so  that  the  pres- 
ent sedimentation-basin  capacity  is  about  equal  to  one  day's 
supply,  or  relatively  less  than  a  third  of  the  original  provision. 
The  idea  followed  is  that  most  of  the  particles  which  will  settle 
at  all  will  do  so  within  twenty-four  hours,  and  that  a  greater 
storage  capacity  may  allow  the  growth  of  algae,  and  that  the 
water  may  deteriorate  rather  than  improve  in  larger  tanks. 

At  London,  on  the  other  hand,  the  authorities  consider  a 
large  storage  capacity  for  unfiltered  water  as  one  of  the  most 
important  conditions  of  successful  filtration,  the  object  however, 
being  perhaps  as  much  to  secure  storage  as  to  allow  sedimenta- 
tion. In  1893  thirty-nine  places  were  reported  upon  the  Thames 
and  the  Lea  which  were  giving  their  sewage  systematic  treat- 
ment before  discharging  it  into  the  strearns-^op^i^ich  London's 

OF    THE  ^ 

UNIVERSITY 


10  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

water  is  drawn.  These  sewage  treatments  are,  with  hardly  an 
exception,  dry-weather  treatments,  and  as  soon  as  there  is  a  con- 
siderable storm  crude  sewage  is  discharged  into  the  rivers  at 
every  point.  The  rivers  are  both  short,  and  are  quickly  flooded,, 
and  afterwards  are  soon  back  in  their  usual  condition.  At  these 
times  of  flood,  the  raw  water  is  both  very  turbid  and  more 
polluted  by  sewage  than  at  other  times,  and  it  is  the  aim  of  the 
authorities  to  have  the  water  companies  provide  reservoir  capac- 
ity enough  to  carry  them  through  times  of  flood  without  draw- 
ing any  water  whatever  from  the  rivers.  This  obviously  involves 
much  more  extensive  reservoirs  than  those  used  in  Germany,  and 
the  companies  actually  have  large  basins  and  are  still  adding 
to  them.  The  storage  capacities  of  the  various  companies  vary 
from  3  to  1 8  times  the  respective  average  daily  supplies,  and 
together  equal  9  times  the  total  supply. 

In  case  the  raw  water  is  taken  from  a  lake  or  a  river  at  a 
point  where  there  is  but  little  current,  as  in  a  natural  or  artificial 
pond,  sedimentation-basins  are  unnecessary.  This  is  the  case  at 
Zurich  (lake  water),  at  Berlin  when  the  rivers  Havel  and  Spree 
spread  into  lakes,  at  Tegel  and  Miiggel,  and  at  numerous  other 
works. 

SIZE   OF   FILTER-BEDS. 

The  total  area  of  filters  required  in  any  case  is  calculated 
from  the  quantity  of  water  required,  the  rate  of  filtration,  and  an 
allowance  for  filters  out  of  use  while  being  cleaned.  To  prevent 
interruptions  of  the  supply  at  times  of  cleaning,  the  filtering  area 
is  divided  into  beds  which  are  operated  separately,  the  number 
and  size  of  the  beds  depending  upon  local  conditions.  The  cost 
per  acre  is  decreased  with  large  beds  on  account  of  there  being 
less  wall  or  embankment  required,  while,  on  the  other  hand,  the 
convenience  of  operation  may  suffer,  especially  in  small  works.  It 
is  also  frequently  urged  that  with  large  filters  it  is  difficult  or  im- 
possible to  get  an  even  rate  of  filtration  over  the  entire  area  ow. 


CONTINUOUS  FILTERS  AND  THEIR  CONSTRUCTION.       n 

ing  to  the  frictional  resistance  of  the  underdrains  for  the  more 
distant  parts  of  the  filter.  A  discussion  of  this  point  is  given  in 
Chapter  III,  page  37.  At  Hamburg,  where  the  size  of  the  single 
beds,  1.88  acres  each,  is  larger  than  at  any  other  place,  it  is  shown 
that  there  is  no  serious  cause  for  anxiety  ;  and  even  if  there  were, 
the  objectionable  resistance  could  be  still  farther  reduced  by  a 
few  changes  in  the  under-drains.  The  sizes  of  filter-beds  used 
at  a  large  number  of  places  are  given  in  Appendix  IV. 

At  a  number  of  places  having  severe  winters,  filters  are 
vaulted  over  as  a  protection  from  cold,  and  in  the  most  important 
of  these,  Berlin,  Warsaw,  and  St.  Petersburg,  the  areas  of  the 
single  beds  are  nearly  the  same,  namely,  from  0.52  to  0.59  acre. 
The  works  with  open  filters  at  London  (seven  companies),  Am- 
sterdam, and  Breslau  have  filter-beds  from  0.82  to  1.50  acres  each. 
Liverpool  and  Hamburg  alone  use  filters  with  somewhat  larger 
areas.  Large  numbers  of  works  with  both  covered  and  open 
filters  have  much  smaller  beds  than  these  sizes,  but  generally  this 
is  to  avoid  too  small  a  number  of  divisions  in  a  small  total  area, 
although  such  works  have  sometimes  been  extended  with  the 
growth  of  the  cities  until  they  now  have  a  considerable  number 
of  very  small  basins. 


FORM   OF  FILTER-BEDS. 

The  form  and  construction  of  the  filter-beds  depend  upon 
local  conditions,  the  foundations,  and  building  materials  available, 
the  principles  governing  these  points  being  in  general  the  same 
as  for  the  construction  of  ordinary  reservoirs.  The  bottoms  re- 
quire to  be  made  water-tight,  either  by  a  thin  layer  of  concrete 
or  by  a  pavement  upon  a  puddle  layer.  For  the  sides  either 
masonry  walls  or  embankments  are  used,  the  former  saving 
space,  but  being  in  general  more  expensive  in  construction. 
Embankments  must,  of  course,  be  substantially  paved  near  the 


12  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

water-line  to  withstand  the  action  of  ice,  and  must  not  be  injured 
by  rapid  fluctuations  in  the  water-levels  in  the  filters. 

Failure  to  make  the  bottoms  water-tight  has  perhaps  caused 
more  annoyance  than  any  other  single  point.  With  a  leaky  bot- 
tom there  is  either  a  loss  of  water  when  the  water  in  the  filters 
is  higher  than  the  ground-water,  or  under  reverse  conditions,  the 
ground-water  comes  in  and  mixes  with  the  filtered  water,  and 
the  latter  is  rarely  improved  and  may  be  seriously  damaged  by 
the  admixture.  And  with  very  bad  conditions  water  may  pass 
from  one  filter  to  another,  with  the  differences  in  pressure  always 
existing  in  neighboring  filters,  with  most  unsatisfactory  results. 


COVERS   FOR  FILTERS. 

The  filters  in  England  and  Holland  are  built  open,  without 
protection  from  the  weather.  In  Germany  the  filters  first  built 
were  also  open,  but  in  the  colder  climates  more  or  less  difficulty 
was  experienced  in  keeping  the  filters  in  operation  in  cold 
weather.  An  addition  to  the  Berlin  filters,  built  in  1874,  was 
covered  with  masonry  vaulting,  over  which  several  feet  of  earth 
were  placed,  affording  a  complete  protection  against  frost.  The 
filters  at  Magdeburg  built  two  years  later  were  covered  in  the 
same  way,  and  since  that  time  covered  filters  have  been  built 
at  perhaps  a  dozen  different  places. 

It  was  found  at  Berlin  that,  owing  to  the  difficulty  of  properly 
cleaning  the  open  filters  in  winter,  it  was  impossible  to  keep  the 
usual  proportion  of  the  area  in  effective  service,  and  as  a  result 
portions  of  the  filters  were  greatly  overtaxed  during  prolonged 
periods  of  cold  weather.  This  resulted  in  greatly  decreased 
bacterial  efficiency,  the  bacteria  in  March,  1889,  reaching  3000  to 
4000  per  cc.  (with  100,000  in  the  raw  water),  although  ordinarily 
the  effluent  contained  less  than  100.  An  epidemic  of  typhoid 
fever  followed/and  was  confined  to  that  part  of  the  city  supplied 


CONTINUOUS  FILTERS  AND    THEIR   CONSTRUCTION.          13 

from  the  Stralau  works,  the  wards  supplied  from  the  covered 
Tegel  filters  remaining  free  from  fever.  Open  filters  have  since 
been  abandoned  in  Berlin. 

At  Altona  also,  where  the  water  is  taken  from  an  excessively 
polluted  source,  decreased  bacterial  efficiency  has  repeatedly 
resulted  in  winter,  and  the  occasional  epidemics  of  typhoid  fever 
in  that  city,  which  have  invariably  come  in  winter,  appear  to  have 
been  directly  due  to  the  effect  of  cold  upon  the  open  niters. 
The  city  has  just  extended  the  open  filters,  and  hopes  with  an 
increased  reserve  area  to  avoid  the  difficulty  in  future  without 
resource  to  covered  filters.  (See  Appendices. II  and  VII.) 

Brunswick,  Liibeck,  and  Frankfort  on  Oder  with  cold  winters 
have  open  filters,  but  draw  their  water-supplies  from  less  pol- 
luted sources,  and  have  thus  far  escaped  the  fate  of  Berlin  and 
Altona.  The  new  filters  at  Hamburg  also  are  open.  At  Zurich, 
where  open  and  covered  filters  were  long  used  side  by  side,  the 
covered  filters  were  much  more  satisfactory,  arid  the  old  open 
filters  have  recently  been  vaulted  over. 

Konigsberg  originally  built  open  filters,  but  was  afterward 
obliged  to  cover  them,  on  account  of  the  severe  winters;  and  at 
Breslau,  where  open  filters  have  long  been  used,  the  recent  ad- 
ditions are  vaulted  over. 

The  fact  that  inferior  efficiency  of  filtration  results  with  open 
filters  during  prolonged  and  severe  winter  weather  is  generally 
admitted,  although  there  is  some  doubt  as  to  the  exact  way  in 
which  the  disturbance  is  caused.  In  some  works  I  am  informed 
that  in  cutting  the  ice  around  the  edges  of  the  filter  and  re- 
peatedly piling  the  loose  pieces  upon  the  floating  cake,  the  latter 
eventually  becomes  so  thickened  at  the  sides  that  the  projecting 
lower  corners  actually  touch  the  sand,  with  the  fluctuating  levels 
which  often  prevail  in  these  works,  and  that  in  this  way  the 
sediment  layer  upon  the  top  of  the  sand  is  broken  and  the  water 
rapidly  passes  without  adequate  purification  at  the  points  of  dis- 
turbance. 


14  FILl^RATION  OF  PUBLIC   WATER-SUPPLIES. 

This  theory  is,  however,  inadequate  to  account  for  many 
cases  where  such  an  accumulation  of  ice  is  not  allowed.  In 
these  cases  the  poor  work  is  not  obtained  until  after  the  niters 
have  been  scraped.  The  sand  apparently  freezes  slightly  while 
the  water  is  off,  and  when  water  is  brought  back  and  nitration 
resumed,  normal  results  are  for  some  reason  not  again  obtained 
for  a  time. 

In  addition  to  the  poorer  work  from  open  filters  in  cold 
weather,  the  cost  of  removing  the  ice  adds  materially  to  the 
operating  expenses,  and  in  very  cold  climates  would  in  itself 
make  covers  advisable. 

I  have  arranged  the  European  filter  plants,  in  regard  to 
which  I  have  sufficient  information,  in  the  table  on  page  15,  in 
the  order  of  the  normal  mean  January  temperatures  of  the 
respective  places.  This  may  not  be  an  ideal  criterion  of  the 
necessity  of  covering  filters,  but  it  is  at  least  approximate,  and  in 
the  absence  of  more  detailed  comparisons  it  will  serve  to  give  a 
good  general  idea  of  the  case.  I  have  not  found  a  single  case 
where  covered  filters  are  used  where  the  January  temperature 
is  32°  F.  or  above.  In  some  of  these  places  some  trouble  is  ex- 
perienced in  unusually  cold  weather,  but  I  have  not  heard  of  any 
very  serious  difficulty  or  of  any  talk  of  covering  filters  at  these 
places  except  at  Rotterdam,  where  a  project  for  covering  was 
being  discussed. 

Those  places  having  January  temperatures  below  30°  experi- 
ence a  great  deal  of  difficulty  with  open  filters  ;  so  much  so,  that 
covered  filters  may  be  regarded  as  necessary  for  them,  although 
it  is  possible  to  keep  open  filters  running  with  decreased  effi- 
ciency and  increased  expense  by  freely  removing  the  ice,  with 
January  temperatures  some  degrees  lower. 

Where  the  mean  January  temperature  is  30°  to  32°  F.  there  is 
room  for  doubt  as  to  the  necessity  of  covering  filters,  but,  judging 
from  the  experience  of  Berlin  and  Altona,  the  covered  filters 
are  much  safer  at  this  temperature. 


CONTINUOUS  FILTERS  AND    THEIR   CONSTRUCTION.          15 
TABLE    OF    PLACES    HAVING    OPEN    AND    COVERED    FILTERS. 
ARRANGED   ACCORDING  TO   THE  MEAN  JANUARY  TEMPERATURES. 


Normal  Mean 

January 

Temperature. 

Degrees  F. 


Place. 


Kind  of  Filters  and  Results. 


37-40° 
33-35° 

32° 


3°o 

30 
30° 
29° 


29° 

29° 

29° 
29° 
26° 


24° 
16° 


All  English  cities 
Cities  in  Holland 


Bremen 
Altona 

Brunswick 

Hamburg 

Lubeck 

Berlin 


Magdeburg 

Frankfort  on  Oder 

Stuttgart 

Stettin 

Zurich 


Liegnitz 
Breslau 

Budapest 

Posen 
Konigsberg 

Warsaw 
St.  Petersburg 


Open  filters  only  are  used,  and  no  great 
difficulty  with  ice  is  experienced. 

All  filters  are  open,  and  there  is  little  se- 
rious trouble  with  ice  ;  but  at  Amster- 
dam and  Rotterdam  the  bacteria  in 
effluents  are  said  to  be  higher  in  winter 
than  at  other  times. 

Open  filters. 

Much  difficulty  with  ice  in  open  filters 
(see  Appendices  II  and  VII). 

Open  filters. 


Open  filters  were  formerly  used,  but  owing 
to  decreased  efficiency  in  cold  weather 
they  have  been  abandoned  for  covered 
ones. 

Covered  filters,  but  a  recent  addition  is 
not  covered. 

Open  filters. 

Part  of  the  filters  are  covered. 

Covered  filters  were  much  the  most  satis- 
factory, and  the  open  ones  were  cov- 
ered in  1894.  The  raw  water  has  a 
temperature  of  35°. 

Open  filters. 

Open  filters  have  been  used,  but  recent 
additions  are  covered. 

Covered  filters  only. 

The  original  filters  were  open,  but  it  was 

found  necessary  to  cover  them. 
Covered  filters  only. 


In  case  the  raw  water  was  drawn  from  a  lake  at  a  depth 
where  its  minimum  temperature  was  above  32°,  which  is  the 
temperature  which  must  ordinarily  be  expected  in  surface-waters 
in  winter,  open  niters  might  be  successfully  used  in  slightly 
colder  places. 

The  covers  are  usually  of  brick  or  concrete  vaulting   sup- 


1 6  FIL7*RAT10N  OF  PUBLIC   WATER-SUPPLIES. 

ported  by  pillars  at  distances  of  u  to  15  feet  in  each  direction, 
the  whole  being  covered  by  2  or  3  feet  of  earth  ;  and  the  top 
can  be  laid  out  as  a  garden  if  desired.  Small  holes  for  the 
admission  of  air  and  light  are  usually  left  at  intervals.  The 
thickness  of  the  masonry  and  the  sizes  of  the  pillars  used  in 
some  of  the  earlier  German  vaultings  are  unnecessarily  great, 
and  some  of  the  newer  works  are  much  lighter.  For  American 
use,  vaulting  like  that  used  for  the  Newton,  Mass.,  covered  reser- 
voir* should  be  amply  strong. 

Roofs  have  been  used  at  Konigsberg,  Posen,  and  Budapest 
instead  of  the  masonry  vaulting.  They  are  cheaper,  but  do  not 
afford  as  good  protection  against  frost,  and  even  with  great  care 
some  ice  will  form  under  them. 

Provision  must  be  made  for  entering  the  niters  freely  to 
introduce  and  remove  sand.  This  is  usually  accomplished  by 
raising  one  section  of  vaulting  and  building  a  permanent  incline 
under  it  from  the  sand  line  to  a  door  above  the  high-water  line 
in  the  filter. 

The  cost  of  building  covered  filters  is  said  to  average  fully 
one  half  more  than  open  filters. 

Among  the  incidental  advantages  of  covered  filters  is  that 
with  the  comparative  darkness  there  is  no  tendency  to  algse 
growths  on  the  filters  in  summer,  and  the  frequency  of  scraping 
is  therefore  somewhat  reduced.  At  Zurich,  in  1892,  where  both 
covered  and  open  filters  were  in  use  side  by  side,  the  periods 
between  scrapings  averaged  a  third  longer  in  the  covered  than 
in  the  open  filters. 

It  has  been  supposed  that  covered  filters  kept  the  water  cool 
in  summer  and  warm  in  winter,  but  owing  to  the  large  volume 
of  water  passing,  the  change  in  temperature  in  any  case  is  very 
slight ;  Fruhling  found  that  even  in  extreme  cases  a  change  of 
over  3°  F.  in  either  direction  is  rarely  observed. 

*  Annual  Report  of  Albert  F.  Noyes,  City  Engineer  for  1891. 


CONTINUOUS  FILTERS  AND    THEIR   CONSTRUCTION. 


At  Berlin,  where  open  and  covered  filters  were  used  side  by 
side  at  Stralau  for  twenty  years,  it  was  found  that,  bacteri- 
ally,  the  open  filters  were,  except  in  severe  winter  weather,  more 
efficient.  It  was  long  supposed  that  this  was  caused  by  the  ster- 
ilizing action  of  the  sunlight  upon  the  water  in  the  open  filters. 
This  result,  however,  was  not  confirmed  elsewhere,  and  it  was 
finally  discovered,  in  1893,  that  the  higher  numbers  were  due  to 
the  existence  of  passages  in  corners  on  the  columns  of  the 
vaulted  roof  and  around  the  ventilators  for  the  underdrains, 
through  which,  practically,  unfiltered  water  found  its  way  into 
the  effluent.  This  at  once  removes  the  evidence  in  favor  of 
the  superior  bacterial  efficiency  of  open  filters  and  suggests  the 
necessity  of  preventing  such  passages.  The  construction  of  a 
ledge  all  around  the  walls  and  pillars  four  inches  wide  and  a 
little  above  the  gravel,  as  shown  in  the  sketch,  might  be  useful 
in  this  way,  and  the  slight  lateral 
movement  of  the  water  in  the 
sand  above  would  be  of  no  con- 
sequence. The  sand  would  evi- 
dently make  a  closer  joint  with 
the  horizontal  ledge  than  with  the 
vertical  wall. 

In  regard  to  the  probable  re- 
quirement or  advisability  of  covers 
for  filters  in  the  United  States,  I 
judge,  from  the  European  experi- 
ence, that  places  having  January 
temperatures  below  the  freezing-  FIG.  2. 

point  will  have  considerable  trouble  from  open  filters,  and  would 
best  have  covered  filters.  Places  having  higher  winter  tempera- 
tures will  be  able  to  get  along  with  the  ice  which  may  form  on 
open  filters,  and  the  construction  of  covers  would  hardly  be  ad- 
visable except  under  exceptional  local  conditions,  as,  for  instance, 
with  a  water  with  an  unusual  tendency  to  algae  growths. 


1 8  FILTRATION   OF  PUBLIC   WATER-SUPPLIES. 

\  have  drawn  a  line  across  a  map  of  the  United  States  on 
this  basis  (shown  by  the  accompanying  plate)  and  it  would  appear 
that  places  far  north  of  the  line  would  require  covered  niters, 
and  that  those  south  of  it  would  not,  while  for  the  places  in  the 
immediate  vicinity  of  the  line  (comparable  to  Hamburg  and 
Altona)  there  is  room  for  discussion. 

In  case  open  niters  are  built  near  or  north  of  this  line,  I 
would  suggest  that  plenty  of  space  between  and  around  the 
niters  for  piling  up  ice  in  case  of  necessity  may  be  found 
advantageous,  and  that  a  greater  reserve  of  filtering  area  for  use 
in  emergencies  should  be  provided  than  would  be  considered 
necessary  with  vaulted  filters  or  with  open  filters  in  a  warmer 
climate. 


Map    showing1 

Normal  Mean   January  Temperatures 

IN  THE  UNITED    STATES 

and   theArea  in  which    Filters    should    be    covered 


51. 

•  SANANTO 


i      I*?.          20.7 
^  DtsMoiMts     OAVEN 


FILTERING   MATERIALS.  1 9 


CHAPTER 

FILTERING  MATERIALS. 
SAND. 

THE  sand  used  for  filtration  may  be  obtained  from  the  sea- 
shore, from  river-beds  or  from  sand-banks.  It  consists  mainly 
of  sharp  quartz  grains,  but  may  also  contain  hard  silicates.  As 
it  occurs  in  nature  it  is  frequently  mixed  with  clayey  or  other 
fine  particles,  which  must  be  removed  from  it  by  washing  before 
it  is  used.  Some  of  the  New  England  sands,  however,  as  that 
used  for  the  Lawrence  City  filter,  are  so  clean  that  washing 
would  be  superfluous. 

The  grain  size  of  the  sand  best  adapted  to  filtration  has  been 
variously  stated  at  from  \  to  i  mm.,  or  from  0.013  to  0.040  inch. 
The  variations  in  the  figures,  however,  are  due  more  to  the  way 
that  the  same  sand  appears  to  different  observers  than  to  actual 
variations  in  the  size  of  sands  used,  which  are  but  a  small  fraction 
of  those  indicated  by  these  figures. 

As  a  result  of  experiments  made  at  the  Lawrence  Experi- 
ment Station  *  we  have  a  standard  by  which  we  can  definitely 
compare  various  sands.  The  size  of  a  sand-grain  is  uniformly 
taken  as  the  diameter  of  a  sphere  of  equal  volume,  regardless 
of  its  shape.  As  a  result  of  numerous  measurements  of  grains 
of  Lawrence  sands,  it  is.  found  that  when  the  diameter,  as 
given  above,  is  i,  the  three  axes  of  the  grain,  selecting  the  longest 
possible  and  taking  the  other  two  at  right  angles  to  it,  are,  on  an 
average,  1.38,  1.05,  and  0.69,  respectively  and  the  mean  diameter 
is  equal  to  the  cube  root  of  their  product. 

*Rept.  Mass.  State  Board  of  Health,  1892,  p.  541.     See  Appendix  III. 


20  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

It  was  also  found  that  in  mixed  materials  containing  particles 
of  various  sizes  the  water  is  forced  to  go  around  the  larger 
particles  and  through  the  finer  portions  which  occupy  the  inter- 
vening spaces,  so  that  it  is  the  finest  portion  which  mainly  deter- 
mines the  character  of  the  sand  for  filtration.  As  a  provisional 
basis  which  best  accounts  for  the  known  facts,  the  size  of  grain 
such  that  10  per  cent  by  weight  of  the  particles  are  smaller  and 
90  per  cent  larger  than  itself,  is  considered  to  be  the  effective  size. 
The  size  so  calculated  is  uniformly  referred  to  in  speaking  of  the 
size  of  grain  in  this  work. 


Overflow 


FIG.  3. — APPARATUS  USED  FOR  MEASURING  THE  FRICTION  OF  WATER  IN  SANDS. 

Another  important  point  in  regard  to  a  material  is  its  degree 
of  uniformity — whether  the  particles  are  mainly  of  the  same 
size  or  whether  there  is  a  great  range  in  their  diameters.  This 
is  shown  by  the  uniformity  coefficient,  a  term  used  to  designate 
the  ratio  of  the  size  of  grain  which  has  60  per  cent  of  the  sample 
finer  than  itself  to  the  size  which  has  10  per  cent  finer  than 
itself. 


FILTERING  MATERIALS. 


21 


The  frictional  resistance  of  sand  to  water  when  closely  packed, 
with  the  pores  completely  filled  with  water  and  in  the  entire 
absence  of  clogging,  was  found  to  be  expressed  by  the  formula 


Fah. 


where  v  is  the  velocity  of  the  water  in  meters  daily  in  a  solid 
column  of  the  same  area   as   that  of  the  sand,  or 
approximately  in  million  gallons  per  acre  daily ; 
c  is  a  constant  factor  which  present  experiments  indicate 

to  be  approximately  loco; 

</«is  the  effective  size  of  sand  grain  in  millimeters ; 
h  is  the  loss  of  head  (Fig.  3) ; 
/  is    the    thickness    of  sand  through  which  the   water 

passes ; 

/  is  the  temperature  (Fahr.). 

The  formula  can  only  be  used  for  sands  with  uniformity 
coefficients  below  5  and  effective  sizes  from  o.io  to  3.00  mm.,  and 
with  the  coarser  materials  only  for  moderately  low  rates.  The 
following  table  shows  the  rates,  in  million  gallons  per  acre  daily, 
at  which  water  will  pass  through  sands  of  different  sizes. 


TABLE  SHOWING   RATE   AT   WHICH   WATER  WILL   PASS  THROUGH 

DIFFERENT-SIZED   SANDS   WITH  VARIOUS   HEADS  AT  A 

TEMPERATURE  OF   50°. 


Effective  Size  in  Millimeters  TO  per  cent  finer  than: 


I 

0.10 

O.2O 

0.30 

o-35 

0.40 

0.50 

I.OO 

3.00 

.001     - 

.005 

.010 
.050 

.01 
.05 
.11 

,C4 

.04 

.21 

•43 
2.14 

Million 
.10 
.48 
•96 

4.82 

Gallons 

•13 
.65 

6.55 

per  Acre 

•17 
.85 
I.7I 

8.55 

daily. 
.27 

1-34 
2.67 
1^.4.0 

1.07 

5-35 
10.70 

Cl.CO 

963 

48.15 
96.30 

.100 

1.  07 

4.28 

Q  6"? 

I  -3    JO 

17  1O 

26  70 

107  oo 

I.OOO 

10.70 

4.2.80 

06  ^o 

131  oo 

I7I.OO 

267  oo 

22  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

RELATIVE   QUANTITIES   OF  WATER   PASSING  AT  DIFFERENT 
TEMPERATURES. 

32° 0.70   44° 0.90    56° 1. 10   68° 1.30 

35°. ...0.75   47°- .-.0.95    59°-..-i.i5   7i°....i.35 

38° 0.80   50° i.oo   62° 1.20   74° 1.40 

41°... .0.85   53°....  1.05   65°....  1.25   77°....  1.45 

SANDS   USED   IN  EUROPEAN  FILTERS. 

To  secure  definite  information  in  regard  to  the  qualities  of 
the  sands  actually  used  in  filtration,  a  large  number  of  European 
works  were  visited  in  1894,  and  samples  of  sand  were  collected 
for  analysis.  These  samples  were  examined  at  the  Lawrence 
Experiment  Station  by  Mr.  H.  W.  Clark,  the  author's  method 
of  analysis  described  in  Appendix  III  being  used.  In  the 
following  table,  for  the  sake  of  compactness,  only  the  leading 
points  of  the  analyses,  namely,  effective  size,  uniformity  coeffi- 
cient, and  albuminoid  ammonia,  are  given.  On  page  26  full 
analyses  of  some  samples  from  a  few  of  the  leading  works  are 
given. 

The  English  and  most  of  the  German  sands  are  washed, 
even  when  entirely  new,  before  being  used,  to  remove  fine 
particles.  At  Breslau,  however,  sand  dredged  from  the  river 
Oder  is  used  in  its  natural  state,  and  new  sand  is  used  for 
replacing  that  removed  by  scraping.  At  Budapest,  Danube  sand 
is  used  in  the  same  way,  but  with  a  very  crude  washing,  and 
it  is  said  that  only  new  unwashed  sand  is  used  at  Warsaw. 

In  Holland,  so  far  as  I  learned,  no  sand  is  washed,  but  new 
sand  is  always  used  for  refilling.  At  most  of  the  works  visited 
dune-sand  with  an  effective  size  of  only  0.17  to  0.19  mm.  is  used, 
and  this  is  the  finest  sand  which  I  have  ever  found  used  for  water 
filtration  on  a  large  scale.  It  should  be  said,  however,  that  the 
waters  filtered  through  these  fine  sands  are  fairly  clear  before 
filtration,  and  are  not  comparable  to  the  turbid  river-waters  often 


FILTERING  MATERIALS. 
ANALYSES   OF   SANDS   USED   IN   WATER   FILTRATION. 


Source. 

Effective 
Size;  io# 
Finer 
than 
(Milli- 
meters). 

Uni- 
formity 
Coeffi- 
cient. 

Albu- 
minoid 
Ammo 
nia. 
Parts  in 

IOO,  OOO 

Remarks. 

London,E.  London  Co. 

«                     « 

"         Grand  June... 
«             «<           « 

«             «           « 
Southw'k&V. 

"       Lambeth  

0.44 
0-39 
0.37 
O.26 
O.40 
0.41 
0.38 
0.30 

o  76 

1.8 
2.1 
2.0 
1-9 

3-5 
3-7 
3-5 
1.8 

2.7 

0.45 

26.20 
8.60 
1.90 
10.00 

2.70 

5.00 
2.80 

2   60 

New  sand,  never  used  or  washed. 
Dirty  sand,  very  old. 
Same,  washed  by  hand. 
Sand  from  rough  filter. 
Old  sand  in  final  filter. 

Freshly  washed  old  sand. 
<«             «          «       « 

Freshly  washed  new  sand. 
Freshly  washed  old  sand. 

41                               « 

wOw 
o  76 

2.4 

0.71; 

New  unused  sand,  washed. 

«                                <( 

0.25 

1.7 

0.70 

New  extremely  fine  sand. 

Chelsea  

0.36 

2.4 

2.10 

Freshly  washed  old  sand. 

Middlesborough  

0.42 

.6 

17.60 

Dirty  sand,  ordinary  scraping. 

0.43 

.6 

7.30 

Same,  after  washing. 

Birmingham 

o  20 

Q 

77    2O 

Dirty  sand. 

« 
Reading  

0.29 
o  30 

•  9 

2  c 

7.20 

4.OO 

Sand  below  surface  of  filter. 
Dirty  sand. 

O   22 

2   O 

I  .  SO 

Same,  after  washing. 

Antwerp  

0    78 

6 

7  80 

Dirty  sand. 

O    7Q 

6 

7    AO 

Same,  after  washing. 

Hamburg  

J^ 

o  28 

2    S 

8  50 

Dirty  sand. 

O    71 

2    7 

o  80 

Same,  after  washing. 

M 

o  74 

2   2 

7  QO 

Dirty  sand,  another  sample. 

ft 

O    7O 

2   O 

O   QO 

Same,  after  washing  drums. 

M 

V.JVf 

O    74. 

2    7 

I     CO 

"         "           "        ejectors. 

Altona  

O    72 

2   O 

Q   OO 

Dirty  sand,  old  filters. 

« 

O    77 

2   o 

I     CO 

Same  after  washing. 

« 

w-  O/ 
O.  77 

2   8 

*  .  jv 

O    CO 

Washed  sand  for  new  filters. 

Berlin,  Stralau..  .  .... 

O    77 

I    O 

12    2O 

Dirty  sand-pile. 

« 

O    7C 

I   7 

4.    SO 

Filter  No.  6,  3"  below  surface. 

« 

O    74 

•7 

<*-  y 
6    7O 

«    7   .<      « 

M 

O   7C 

7 

4OO 

«        ««  IO  «      «           tt 

Tegel  

o  78 

6 

1  1    OO 

Dirty  sand,  old  filters. 

V.   JV 

o  78 

c 

2    80 

Same,  after  washing,  old  filters. 

<i 

O    7C 

:! 

7    2O 

«          •«          «<          new    « 

Miiggel.   .     . 

v-  OJ 
O    7<J 

8 

o  80 

Sand  from  filters  below  surface. 

O    77 

2   O 

6  70 

Dirty  sand,  ordinary  scraping. 

«< 

O    74 

2   O 

ic    70 

"         "      another  sample. 

Charlottenburg  

O   4.O 

2    7 

7   2O 

«        « 

Chemnitz  

O    7? 

2   6 

o  20 

New  sand  not  yet  used. 

Magdeburg  

O    7Q 

2   O 

9    CO 

Dirty  sand 

**•  jy 

O   AO 

2   o 

•  Dv 

2    80 

Same  after  washing 

Breslau  

O    70 

I    8 

I    4.O 

Normal  new  sand. 

Budapest  

O   2O 

2   O 

o  80 

New  washed  Danube  sand 

Zurich  

o  28 

32 

6  20 

Dirty  sand 

O    7O 

•  * 
31 

I    CO 

Same  after  washing. 

Hague.  .  . 

O    IQ 

I  6 

o  70 

Dune-sand  used  for  filtration. 

v.  *y 

24  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

ANALYSES   OF  SANDS   USED   IN  WATER  FILTRATION.— Continued. 


Source. 

Effective 
Size;  io# 
Finer 
than 
(Milli- 
meters). 

Uni- 
formity 
Coeffi- 
cient. 

Albu- 
minoid 
Ammo- 
nia. 
Parts  in 

100,000. 

Remarks. 

Schiedam  

0.18 

1.6 

5.60 

Dune-sand    used    for    filtration  • 

« 

0.31 

i.? 

M.  ?0 

dirty. 
River-sand  ;  dirty. 

Amsterdam  

0.17 

1.6 

2.4O 

Dune-sand. 

Rotterdam  

0.34 

i.5 

2.30 

River-sand  ;  new. 

Liverpool,  Rivingtori.. 
«                 « 
44       Owcsty  

0.43 
0.32 

0-43 
o  30 

2.O 
2.5 

2.7 

2  6 

0.76 
I.OO 

4.10 
0  4O 

Sand  from  bottom  of  filter. 
New    sand    unwashed    and    un- 
screened. 
Washed  sand  which  has  been  in 
use  30  to  40  years. 
Dirty  sand. 

O.  "U 

4.7 

2   2O 

Same,  after  washing 

NOTE. — It  is  obvious  that  in  case  the  sands  used  at  any  place  are  not  always  of  the 
same  character,  as  is  shown  to  be  the  case  by  different  samples  from  some  of  the  works, 
the  examination  of  such  a  limited  number  of  samples  as  the  above  from  each  place  is 
entirely  inadequate  to  establish  accurately  the  sizes  of  sand  used  at  that  particular 
place,  or  to  allow  close  comparisons  between  the  different  works,  and  for  this  reason  no 
such  comparisons  will  be  made.  The  object  of  these  investigations  was  to  determine 
the  sizes  of  the  sands  commonly  used  in  Europe,  and,  considering  the  number  and 
character  of  the  different  works  represented,  it  is  believed  that  the  results  are  ample 
for  this  purpose. 

filtered  elsewhere,  and  their  tendency  to  choke  the  filters  is  con- 
sequently much  less.  At  Rotterdam  and  Schiedam,  where  the 
raw  water  is  drawn  from  the  Maas,  as  the  principal  stream  of  the 
Rhine  is  called  in  Holland,  river-sand  of  much  larger  grain  size 
is  employed.  It  is  obtained  by  dredging  in  the  river  and  is  never 
washed,  new  sand  always  being  employed  for  refilling. 

The  average  results  of  the  complete  analyses  of  sands  from 
ten  leading  works  are  shown  in  the  table  on  page  26.  These 
figures  are  the  average  of  all  the  analyses  for  the  respective 
places,  except  that  one  sample  from  the  Lambeth  Co.,  which  was 
not  a  representative  one,  was  omitted. 

The  London  companies  were  selected  for  this  comparison 
both  on  account  of  their  long  and  favorable  records  in  filtering 


FILTERING   MATERIALS.  2$ 

the  polluted  waters  of  the  Thames  and  Lea,  and  because  they  are 
subject  to  close  inspection ;  and  there  is  ample  evidence  that  the 
filtration  obtained  is  good — evidence  which  is  often  lacking  in  the 
smaller  and  less  closely  watched  works.  For  the  German  works 
Altona  was  selected  because  of  its  escape  from  cholera  in  1892, 
due  to  the  efficient  action  of  its  filters,  and  Stralau  because  of  its 
long  and  favorable  record  when  filtering  the  much-polluted  Spree 
water.  These  two  works  also  have  perhaps  contributed  more 
to  the  modern  theories  of  filtration  than  all  the  other  works  in 
existence.  The  remaining  works  are  included  because  they  are 
comparatively  new,  and  have  been  constructed  with  the  greatest 
care  and  attention  to  details  throughout,  and  the  results  obtained 
are  most  carefully  recorded. 

The  averages  show  the  effective  size  of  the  English  sands  to 
be  slightly  greater  than  that  of  the  German  sands — 0.37  instead  of 
0.34  mm. — but  the  difference  is  very  small.  The  entire  range 
for  the  ten  works  is  only  from  0.31  to  0.40  mm.,  and  these  may 
be  taken  as  the  ordinary  limits  of  effective  size  of  the  sands  em- 
ployed in  the  best  European  works.  The  average  for  the  other 
sixteen  works  given  above,  including  dune-sands,  is  0.31  mm.,  or, 
omitting  the  dune-sands,  0.34  mm. 

Turning  to  the  circumstances  which  influence  the  selection 
of  the  sand  size,  we  find  that  both  the  quality  of  the  effluent  ob- 
tained by  filtration  and  the  cost  of  filtration  depend  upon  the 
size  of  the  sand-grains. 

With  a  fine  sand  the  sediment  layer  forms  more  quickly  and 
the  removal  of  bacteria  is  more  complete,  but,  on  the  other  hand, 
the  filter  clogs  quicker  and  the  dirty  sand  is  more  difficult  to 
wash,  so  that  the  expense  is  increased. 

EFFECT  OF  SIZE  OF  GRAIN  UPON  EFFICIENCY  OF  FILTRATION. 

It  is  frequently  stated  that  it  is  only  the  sediment  layer 
which  performs  the  work  of  filtration,  and  that  the  sand  which 
supports  it  plays  hardly  a  larger  part  than  does  the  gravel  which 


26 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


TABLE    SHOWING  THE  AVERAGE  PER  CENT  OF    THE  GRAINS   FINER 
THAN  VARIOUS   SIZES  IN  SANDS   FROM   LEADING  WORKS. 


Per  Cent  by  Weight  Finer  than 

0.106 
mm. 

0.186 
mm. 

0.316 
mm. 

0.46 

mm. 

o-93 
mm. 

2.04 

mm. 

3  89 
mm. 

5-89 

mm. 

East  London  

0.2 

o 

O 
O 
0.2 
O.I 

o.5 

O.2 

o.7 

0.5 

O.I 

1.5 
i.i 

0.3 

O.2 

o.5 

3.6 

3-1 
8.0 

5-5 
5-0 
10.9 
7.8 
7-0 
4-5 
7-9 

22.2 
17.4 

34.1 

26.6 
28.6 

33.2 

28.7 
37-3 
35-4 

33.6 

69.7 

47.1 
69.7 
63.0 
63.0 

74-4 
72.1 
86.9 
94-3 
79-7 

89.8 
68.2 
83.5 

79.2 
76.7 
95-7 
92.1 

95.4 
98.5 
94-3 

95-0 
84.7 
90.0 
88.0 
86.0 

99-5 
95.8 

97.6 

99-0 
93-6 
94-0 
94-3 
93-6 

Grand  Junction 

Southwark  and  Vauxhall  . 
Lambeth  

Chelsea  

Hamburg  

Tegel        

Miiggel  

O.I 

Average  of  all  

O.O6 

0.56 

6.33 

29.71 

71.99 

87.34 

93-42 

(97-45) 

AVERAGE  EFFECTIVE  SIZE,   UNIFORMITY  COEFFICIENT,  AND  ALBU- 
MINOID AMMONIA  IN  SANDS  FROM  TEN  LEADING  WORKS. 

I.      LONDON   FILTERS. 


Effective 
Size;  io# 

Uniformity 

Albuminoic 

I  Ammonia. 

Finer  than 
(Millimeters). 

Coefficient. 

Dirty  Sand. 

Washed  Sand. 

East  London  

o  4.0 

2   O 

26  oo 

8  60 

Grand  Junction  

o  4.0 

3.6 

IO.OO 

2    70 

O   34 

2    C 

0     QO 

•  ->7 
o  36 

2   4. 

2    60 

Chelsea      .   

\j.  yj 
o  36 

2   4 

2    IO 

O   37 

2   6 

18  oo 

3   08 

II.     GERMAN   WORKS. 


Stralau  

0.34 

1.7 

1  2.  2O 

4.OO 

Tegel                  

o  37 

i  6 

I  I    OO 

3   OO 

Miteffel.  . 

o.  34 

2.O 

10.80 

0.80 

0.34 

2.3 

9.00 

I.  50 

o  31 

2    3 

8    20 

I    O7 

o.  34 

2.0 

10.21; 

2.O7 

FILTERING  MATERIALS.  2J 

carries  the  sand,  and  under  some  circumstances  this  is  un- 
doubtedly the  case.  Nevertheless  sand  in  itself,  without  any 
sediment  layer,  especially  when  not  too  coarse  and  not  in  too 
thin  layers,  has  very  great  purifying  powers,  and,  in  addition, 
acts  as  a  safeguard  by  positively  preventing  excessive  rates  of 
filtration  on  account  of  its  frictional  resistance.  As  an  illustration 
take  the  case  of  a  filter  of  sand  with  an  effective  size  of  0.35  mm. 
and  the  minimum  thickness  of  sand  allowed  by  the  German  Board 
of  Health,  namely,  one  foot,  and  let  us  suppose  that  with  clogging 
the  loss  of  head  has  reached  two  feet  to  produce  the  desired  veloc- 
ity of  2.57  million  gallons  per  acre  daily.  Suppose  now  that  by 
some  accident  the  sediment  layer  is  suddenly  broken  or  removed 
from  a  small  area,  the  water  will  rush  through  this  area,  until  a 
new  sediment  layer  is  formed,  at  a  rate  corresponding  to  the  size, 
pressure,  and  depth  of  the  sand,  or  260  million  gallons  per  acre 
daily — a  hundred  times  the  standard  rate.  Under  these  condi- 
tions the  passing  water  will  not  be  purified,  but  will  pollute  the 
entire  effluent  from  the  filter.  Under  corresponding  conditions, 
with  a  deep  filter  of  fine  sand,  say  with  an  effective  size  of  0.20 
mm.  and  5  feet  deep,  the  resulting  rate  would  be  only  17  million 
gallons  per  acre  daily,  or  less  than  seven  times  the  normal,  and 
with  the  water  passing  through  the  full  depth  of  fine  sand,  the 
resulting  deterioration  in  the  effluent  before  the  sand  again  be- 
came so  clogged  as  to  reduce  the  rate  to  nearly  the  normal, 
would  be  hardly  appreciable. 

The  results  at  Lawrence  have  shown  that  with  very  fine  sands 
0.09  and  0.14  mm.,  and  4  to  5  feet  deep,  with  the  quantity  of  water 
which  can  practically  be  made  to  pass  through  them,  it  is  almost 
impossible  to  drive  more  than  an  insignificant  fraction  of  the 
bacteria  into  the  effluent.  Even  when  the  sands  are  entirely 
new,  or  have  been  scraped  or  disturbed  in  the  most  violent  way, 
the  first  effluent  passing,  before  the  sediment  layer  could  have 
been  formed,  is  of  good  quality.  Still  finer  materials,  0.04  to 
0.06  mm.,  as  far  as  could  be  determined,  secured  the  absolute 


28  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

removal  of  all  bacteria,  but  the  rates  of  filtration  which  were 
possible  were  so  low  as  to  preclude  their  practical  application. 

With  coarser  sands,  as  long  as  the  filter  is  kept  at  a  steady 
rate  of  filtration,  without  interruptions  of  any  kind,  entirely  satis- 
factory results  are  often  obtained,  although  never  quite  so  good 
as  with  the  finer  sands.  Thus  at  Lawrence  the  percentages  of 
bacteria  (B.  prodigiosus)  appearing  in  the  effluents  under  compar- 
able conditions  were  as  follows : 

1892          1893 

With  effective  grain  size  0.38  mm 0.16 

"     0.29    "    0.16 

"     0.26    "    o.io 

"       "     0.20    "    0.13  o.oi 

"  "       "     0.14    "    0.04  0.03 

"       0.09      "     0.02  0.02 

We  may  thus  conclude  that  fine  sands  give  normally  some- 
what better  effluents  than  coarser  ones,  and  that  they  are  much 
more  likely  to  give  at  least  a  tolerably  good  purification  under 
unusual  or  improper  conditions. 

EFFECT  OF  GRAIN  SIZE   UPON  FREQUENCY  OF  SCRAPING. 

The  practical  objection  to  the  use  of  fine  sand  is  that  it 
becomes  rapidly  clogged,  so  that  filters  require  to  be  scraped  at 
shorter  intervals,  and  the  sand  washing  is  much  more  difficult 
and  expensive.  The  quantities  of  water  filtered  between  succes- 
sive scrapings  at  Lawrence  in  millions  of  gallons  per  acre  under 
comparable  conditions  have  been  as  follows: 

1892       1893 

Effective  size  of  sand  grain  0.38  mm... , 79 

"     "      "        "       o.29mm 70 

"    "      "        "       o.26mm 57 

"          "     "      "        "       0.20  mm 58        

"          "     "      "        "       0.14  mm 45        49 

"          "     "      "        "       o.09mm 24        14 


FILTERING  MATERIALS.  29 

The  increase  in  the  quantities  passed  between  scrapings  with 
increasing  grain  size  is  very  marked. 

With  the  fine  sands,  the  depth  to  which  the  sand  becomes 
dirty  is  much  less  than  with  the  coarse  sands,  but  as  it  is  not 
generally  practicable  to  remove  a  layer  of  sand  less  than  about 
0.6  inch  thick,  even  when  the  actual  clogged  layer  is  thinner 
than  this,  the  full  quantity  of  sand  has  to  be  removed ;  and 
the  quantities  of  sand  to  be  removed  and  washed  are  inverse- 
ly proportional  to  the  quantities  of  water  filtered  between 
scrapings.  On  the  other  hand,  with  very  coarse  sands  the 
sediment  penetrates  the  sand  to  a  greater  depth  than  the  0.6 
inch  necessarily  removed,  so  that  a  thicker  layer  of  sand  has  to 
be  removed,  which  may  more  than  offset  the  longer  interval. 
This  happens  occasionally  in  water-works,  and  a  sand  coarse 
enough  to  allow  it  occur  is  always  disliked  by  superintendents, 
and  is  replaced  with  finer  sand  as  soon  as  possible.  It  is  ob- 
vious that  the  minimum  expense  for  cleaning  will  be  secured 
with  a  sand  which  just  does  not  allow  this  deep  penetration,  and 
I  am  inclined  to  think  that  the  sizes  of  the  sands  in  use  have  actu- 
ally been  determined  more  often  than  otherwise  in  this  way,  and 
that  the  coarsest  samples  found,  having  effective  sizes  of  about 
0.40  mm.,  represent  the  practical  limit  to  the  coarseness  of  the 
sand,  and  that  any  increase  above  this  size  would  be  followed 
by  increased  expense  for  cleaning  as  well  as  by  decreased 
efficiency. 

SELECTION  OF  SAND. 

In  selecting  a  sand  for  filtration,  when  it  is  considered  that 
repeated  washings  will  remove  some  of  the  finest  particles,  and 
so  increase  slightly  the  effective  size,  a  new  sand  coarser  than 
0.35  mm.  would  hardly  be  selected.  Perhaps  0.20  might  be 
given  as  a  suitable  lower  limit.  For  comparatively  clear  lake-  or 
reservoir-waters  a  finer  sand  could  probably  be  used  than  would 
be  the  case  with  a  turbid  river-water.  A  mixed  sand  having  a 


30  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

uniformity  coefficient  above  3.0  would  be  difficult  to  wash  without 
separating  it  into  portions  of  different  sizes,  and,  in  general,  the 
lower  the  coefficient,  that  is,  the  more  uniform  the  grain  sizes,  the 
better.  Great  pains  should  be  taken  to  have  the  sand  of  the 
same  quality  throughout,  especially  in  the  same  filter,  as  any 
variations  in  the  grain  sizes  would  lead  to  important  variations 
in  the  velocity  of  filtration,  the  coarser  sands  passing  more  than 
their  share  of  water  (in  proportion  to  the  square  of  the  effective 
sizes)  and  with  reduced  efficiency. 

At  Lawrence  a  sufficient  quantity  of  natural  sand  was  found 
of  the  grade  required  ;  but  where  suitable  material  cannot  be  so 
obtained  it  is  necessary  to  use  other  methods.  A  mixed  mate- 
rial can  be  screened  from  particles  which  are  too  large,  and  can 
be  washed  to  free  it  from  its  finer  portions,  and  in  this  way  a 
good  sand  can  be  prepared,  if  necessary,  from  what  might  seem 
to  be  quite  unpromising  material.  The  methods  of  sand-wash- 
ing will  be  described  in  Chapter  V. 

THICKNESS  OF  THE  SAND   LAYER. 

The  thickness  of  the  sand  layer  is  made  so  great  that  when 
it  is  repeatedly  scraped  in  cleaning  the  sand  will  not  become  too 
thin  for  good  filtration  for  a  considerable  time.  When  this 
occurs  the  removed  sand  must  be  replaced  with  clean  sand.  The 
original  thickness  of  the  sand  in  European  filters  is  usually  from 
24  to  48  inches,  thicknesses  between  30  and  40  inches  being  ex- 
tremely common,  and  this  is  reduced  before  refilling  to  from  12 
to  24  inches.  The  Imperial  Board  of  Health  of  Germany  has 
fixed  12  inches  as  a  limit  below  which  the  sand  should  never 
be  scraped,  and  a  higher  limit  is  recommended  wherever  pos- 
sible. 

A  thick  sand  layer  has  the  same  steadying  action  as  a  fine 
sand,  and  tends  to  prevent  irregularities  in  the  rate  of  filtration 
in  proportion  to  its  frictional  resistance,  and  that  without  in- 
creasing the  frequency  of  cleaning ;  but,  on  the  other  hand,  it  in- 


FILTERING   MATERIALS.  31 

creases  the  necessary  height  of  the  filter,  throughout,  and  conse- 
quently the  cost  of  construction. 

In  addition  to  the  steadying  effect  of  a  deep  sand  layer,  some 
purification  takes  place  in  the  lower  part  of  the  sand  even  with 
a  good  sediment  layer  on  the  surface,  and  the  efficiency  of  deep 
filters  is  .greater  than  that  of  shallow  ones. 

Layers  of  finer  materials,  as  fine  sand  or  loam,  in  the  lower 
part  of  a  filter,  which  would  otherwise  give  increased  efficiency 
without  increasing  the  operating  expenses,  cannot  be  used. 
Their  presence  invariably  gives  rise  sooner  or  later  to  sub-sur- 
face clogging  at  the  point  of  junction  with  the  coarser  sand,  as 
has  been  found  by  repeated  tests  at  Lawrence  as  well  as  in  some 
of  the  Dutch  filters  where  such  layers  were  tried ;  and  as  there 
is  no  object  in  putting  a  coarser  sand  under  a  finer,  the  filter  sand 
is  best  all  of  the  same  size  and  quality  from  top  to  bottom. 

UNDERDRAINING. 

The  underdrains  of  a  filter  are  simply  useful  for  collecting 
the  filtered  water ;  they  play  no  part  in  the  purification.  One  of 
the  first  requirements  of  successful  filtration  "is  that  the  rate  of 
filtration  shall  be  practically  the  same  in  all  parts  of  the  filter. 
This  is  most  difficult  to  secure  when  the  filter  has  just  been 
cleaned  and  the  friction  of  the  sand  layer  is  at  a  minimum.  If 
the  friction  of  the  water  in  entering  and  passing  through  the 
underdrains  is  considerable,  the  more  remote  parts  of  the  filters 
will  work  under  less  pressure,  and  will  thus  do  less  than  their 
share  of  the  work,  while  the  parts  near  the  outlet  will  be  over- 
taxed, and  filtering  at  too  high  rates  will  yield  poor  effluents. 

To  avoid  this  condition  the  underdrains  must  have  such  a 
capacity  that  their  frictional  resistance  will  be  only  a  small  frac- 
tion of  the  friction  in  the  sand  itself  just  after  cleaning. 

GRAVEL   LAYERS. 

The  early  filters  contained  an  enormous  quantity  of  gravel, 
but  the  quantity  has  been  steadily  reduced  in  successive  plants. 


32  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

Thus  in  1866  Kirk  wood,  as  a  result  of  his  observations,  recom- 
mended the  use  of  a  layer  four  feet  thick,  and  in  addition  a  foot 
of  coarse  sand,  while  at  the  present  time  new  filters  rarely  have 
more  than  two  feet  of  gravel.  Even  this  quantity  seems  quite 
superfluous,  when  calculations  of  its  f rictional  resistance  are  made. 
Thus  a  layer  of  gravel  with  an  effective  size  of  20  mm.*  (which  is 
much  finer  than  that  generally  employed)  only  6  inches  thick  will 
carry  the  effluent  from  a  filter  working  at  a  rate  of  2.57  million 
gallons  per  acre  daily  for  a  distance  of  8  feet  (that  is,  with  under- 
drains  16  feet  apart),  with  a  loss  of  head  of  only  o.ooi  foot,  and 
for  longer  distances  tile  drains  are  cheaper  than  gravel.  To  pre- 
vent the  sand  from  sinking  into  the  coarse  gravel,  intermediate 
sizes  of  gravel  must  be  placed  between,  each  grade  being  coarse 
enough  so  that  there  is  no  possibility  of  its  sinking  into  the  layer 
below.  The  necessary  thickness  of  these  intermediate  layers  is 
very  small,  the  principal  point  being  to  have  a  layer  of  each  grade 
at  every  point.  Thus  on  the  6  inches  of  20  mm.  gravel  men- 
tioned above,  three  layers  of  two  inches  each,  of  8  and  3  mm. 
gravel  and  coarse  sand,  with  a  total  height  of  six  inches,  or  other 
corresponding  and  convenient  depths  and  sizes,  would,  if  carefully 
placed,  as  effectually  prevent  the  sinking  of  the  filter  ^and  into 
the  coarse  gravel  as  the  much  thicker  layers  used  in  the  older 
plants. 

The  gravel  around  the  drains  should  receive  special  attention. 
Larger  stones  can  be  here  used  with  advantage,  taking  care  tha£ 
adequate  spaces  are  left  for  the  entrance  of  the  water  into  the 
drains  at  a  low  velocity,  and  to  make  everything  so  solid  in  this 
neighborhood  that  there  will  be  no  chance  for  the  stones  to 
settle  which  might  allow  the  sand  to  reach  the  drains. 

At  the  Lawrence  filter,  at  Konigsberg  in  Prussia,  at  Amster- 
dam and  other  places,  the  quantity  of  gravel  is  reduced  by  put- 
ting the.  drains  in  trenches,  so  that  the  gravel  is  reduced  from 

*  The  method  of  calculating  the  size  is  given  in  Appendix  III. 


FILTERING   MATERIALS.  33 

a  maximum  thickness  at  the  drain  to  nothing  half  way  between 
drains.  The  economy  of  the  arrangement,  however,  as  far  as 
friction  is  concerned  is  not  so  great  as  would  appear  at  first  sight, 
and  the  cost  of  the  bottom  may  be  increased  ;  but  on  the  other 
hand  it  gives  a  greater  depth  of  gravel  for  covering  the  drains 
with  a  small  total  amount  of  gravel. 

As  even  a  very  small  percentage  of  fine  material  is  capa- 
ble of  getting  in  the  narrow  places  and  reducing  the  carrying 
power  of  the  gravel,  it  is  important  that  all  such  matters  should 
be  carefully  removed  by  washing  before  putting  the  gravel  in 
place.  In  England  and  Germany  gravel  is  commonly  screened 
for  use  in  revolving  cylinders,  of  wire-cloth  of  the  desired  sizes, 
on  which  water  is  freely  played  from  numerous  jets,  thus  secur- 
ing perfectly  clean  gravel.  In  getting  gravel  for  the  Lawrence 
filter,  an  apparatus  was  used,  in  which  advantage  was  taken  of 
the  natural  slope  of  the  gravel  bank  to  do  the  work,  and  the  use 
of  power  was  avoided.  The  respective  grades  of  gravel  obtained 
were  even  in  size,  and  reasonably  free  from  fine  material,  but  it 
was  deemed  best  to  wash  them  with  a  hose  before  putting  them 
in  the  filter. 

To  calculate  the  frictional  resistance  of  water  in  passing 
gravel,  we  may  assume  that  for  the  very  low  velocities  which 
are  actually  found  in  filters  the  quantity  of  water  passing  varies 
directly  with  the  head,  which  for  these  velocities  is  substantially 
correct,  although  it  would  not  be  true  for  higher  rates,  especially 
with  the  coarser  gravels.*  In  the  case  of  parallel  underdrains  the 
friction  from  the  middle  point  between  drains  to  the  drains  may 
be  calculated  by  the  formula  : 


T  t  1  h     d  —  T  ^ate  °^  filtrati°n  X  (k  distance  between  drains)1 
2  Average  depth  of  gravel  X  discharge  coefficient* 

The  discharge  coefficient  for  any  gravel  is  1000  times  the  quan- 

*  A  full  table  of  frictions  with  various  velocities  and  gravels  was  given  in  the  Rept. 
of  Mass.  State  Board  of  Health,  1892,  p.  555. 


34  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

tity  of  water  which  will  pass  when  -  is  y^Vo  expressed  in  million 

gallons  per  acre  daily.     The  approximate  values  of  this  coefficient 
for  different-sized  gravels  are  as  follows : 

VALUES   OF   DISCHARGE    COEFFICIENT. 

For  gravel  with  effective  size    5  mm c  —    23,000 

"          "        "            "         "  10  "  c=    65,000 

"         "  15  "  c  =  110,000 

"  20  "  c  =  160,000 

"  25  "  c  =  230,000 

"          "        "            "          "  30  "  c  =  300,000 

"  35  "  £  =  390,000 

"  40  "  £  =  480,000 

Example :  What  is  the  loss  of  head  in  the  gravel  at  a  rate  of 
filtration  of  2  million  gallons  per  acre  daily,  with  underdrains  20 
feet  apart,  where  the  supporting  gravel  has  an  effective  size  of  35 
millimeters,  and  is  uniformly  i  ft.  deep  ? 

Total  head  =  -      2  X  IO'     =  .000256  ft. 

2    I  X  390.000 

The  total  friction  would  be  the  same  with  the  same  average 
depth  of  gravel  whether  it  was  uniformly  i  foot  deep,  or  decreas- 
ing from  1.5  at  the  drains  to  0.5  in  the  middle,  or  from  2.0  to  o. 
The  reverse  case  with  the  gravel  layer  thicker  in  the  middle  than 
at  jthe  drains  does  not  occur  and  need  not  be  discussed. 

The  depth  of  gravel  likely  to  be  adopted  as  a  result  of  this 
calculation,  when  the  drains  are  not  too  far  apart,  will  be  much 
less  than  that  actually  used  in  most  European  works,  but  as  the 
two  feet  or  more  there  employed  are,  I  believe,  simply  the  result 
of  speculation,  there  is  no  reason  for  following  the  precedent 
where  calculations  show  that  a  smaller  quantity  is  adequate. 

The  reason  for  recommending  a  thin  lower  layer  of  coarse 
gravel,  which  alone  is  assumed  to  provide  for  the  lateral  move- 


FILTERING   MATERIALS.  35 

ment  of  the  water,  is  that  if  more  than  about  six  inches  of  gravel 
is  required  to  give  a  satisfactory  resistance,  it  will  almost  always 
be  cheaper  to  use  more  drains  instead  of  more  gravel ;  and  the 
reason  for  recommending  thinner  upper  layers  for  preventing  the 
sand  from  settling  into  the  coarse  gravel  is  that  no  failures  of 
this  portion  of  niters  are  on  record,  and  in  the  few  instances 
where  really  thin  layers  have  been  used  the  results  have  been 
entirely  satisfactory.  In  Konigsberg  filters  were  built  by  Friih- 
ling,*  in  which  the  sand  was  supported  by  five  layers  of  gravel  of 
increasing  sizes,  respectively  1.2,  1.2,  1.6,  2.0,  3.2,  or,  together,  9.2 
inches  thick,  below  which  there  were  an  average  of  five  inches  of 
coarse  gravel.  These  were  examined  after  eight  years  of  oper- 
ation and  found  to  be  in  perfect  order. 

At  the  Lawrence  Experiment  Station  filters  have  been  re- 
peatedly constructed  with  a  total  depth  of  supporting  gravel 
layers  not  exceeding  six  inches,  and  among  the  scores  of  such 
filters  there  has  not  been  a  single  failure,  and  so  far  as  they  have 
been  dug  up  there  has  never  been  found  to  have  been  any  move- 
ment whatever  of  the  sand  into  the  gravel.  The  Lawrence  city 
filter,  built  with  corresponding  layers,  has  shown  no  signs  of  be- 
ing inadequately  supported.  In  arranging  the  Lawrence  gravel 
layers  care  has  always  been  taken  that  no  material  should  rest 
on  another  material  more  than  three  or  four  times  as  coarse  as 
itself,  and  that  each  layer  should  be  complete  at  every  point,  so 
that  by  no  possibility  could  two  layers  of  greater  difference  in 
size  come  together.  And  it  is  believed  that  if  this  is  carefully 
attended  to,  no  trouble  need  be  anticipated,  however  thin  the 
single  layers  may  be. 

UNDERDRAINS. 

The  most  common  arrangement,  in  other  than  very  small 
filters,  is  to  have  a  main  drain  through  the  middle  of  the  filter, 

*  Friihling,  Handbuch  der  Ingenieurwissenschaften,  II.  Band,  VI.  Kapitel. 


36  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

with  lateral  drains  at  regular  intervals  from  it  to  the  sides.  The 
sides  of  the  main  drain  are  of  brick,  laid  with  open  joints  to  ad- 
mit water  freely,  and  the  top  is  usually  covered  with  stone  slabs. 
The  lateral  drains  may  be  built  in  the  same  way,  but  tile  drains 
are  also  used  and  are  cheaper.  Care  must  be  taken  with  the  latter 
that  ample  openings  are  left  for  the  admission  of  water  at  very 
low  velocities.  It  is  considered  desirable  to  have  these  drains 
go  no  higher  than  the  top  of  the  coarsest  gravel ;  and  this  will 
often  control  the  depth  of  gravel  used.  If  they  go  higher,  the 
top  must  be  made  tight  to  prevent  the  entrance  of  the  fine 
gravels  or  sand.  Sometimes  they  are  sunk  in  part  or  wholly 
(especially  the  main  drain)  below  the  floor  of  the  filter.  With 
gravel  placed  in  waves,  that  is,  thicker  over  the  drains  than  else- 
where, as  mentioned  above,  the  drains  are  covered  more  easily 
than  with  an  entirely  horizontal  arrangement.  When  this  is  done, 
the  floor  of  the  filter  is  trenched  to  meet  the  varying  thickness 
of  gravel,  so  that  the  top  of  the  latter  is  level,  and  the  sand  has  a 
uniform  thickness. 

Many  filters  (Lambeth,  Brunswick,  etc.)  are  built  with  a 
double  bottom  of  brick,  the  upper  layer  of  which,  with  open  joints, 
supports  the  gravel  and  sand,  and  is  itself  supported  by  numerous 
small  arches  or  other  arrangements  of  brick,  which  serve  to 
carry  the  water  to  the  outlet  without  other  drains.  This  ar- 
rangement allows  the  use  of  a  minimum  quantity  of  gravel,  but  is 
undoubtedly  more  expensive  than  the  usual  form,  with  only  the 
necessary  quantity  of  gravel ;  and  I  am  unable  to  find  that  it  has 
any  corresponding  advantages. 

The  frictional  resistance  of  underdrains  requires  to  be  care- 
fully calculated ;  and  in  doing  this  quite  different  standards  must 
be  followed  from  those  usually  employed  in  determining  the 
sizes  of  water-pipes,  as  a  total  frictional  resistance  of  only  a  few 
hundredths  of  a  foot,  including  the  velocity  head,  may  cause 
serious  irregularities  in  the  rate  of  filtration  in  different  parts  of 
the  filter. 


FILTERING   MATERIALS.  37 

The  sizes  of  the  underdrains  differ  very  widely  in  proportion 
to  the  sizes  of  the  filters  in  European  works,  some  of  them  being 
excessively  large,  while  in  other  cases  they  are  so  small  as  to 
suggest  a  doubt  as  to  their  allowing  uniform  rates  of  filtration, 
especially  just  after  cleaning. 

I  would  suggest  the  following  rules  as  reasonably  sure  to  lead 
to  satisfactory  results  without  making  an  altogether  too  lavish 
provision :  In  the  absence  of  a  definite  determination  to  run 
filters  at  some  other  rate,  calculate  the  drains  for  the  German 
standard  rate  of  a  daily  column  of  2.40  meters,  equal  to  2.57 
million  gallons  per  acre  daily.  This  will  insure  satisfactory 
work  at  all  lower  rates,  and  no  difficulty  on  account  of  the 
capacity  of  the  underdrains  need  be  then  anticipated  if  the  rate 
is  somewhat  exceeded.  The  area  for  a  certain  distance  from 
the  main  drain  depending  upon  the  gravel  may  be  calculated  as 
draining  directly  into  it,  provided  there  are  suitable  openings, 
and  the  rest  of  the  area  is  supposed  to  drain  to  the  nearest  lateral 
drain. 

In  case  the  laterals  are  round-tile  drains  I  would  suggest  the 
following  limits  to  the  areas  which  they  should  be  allowed  to 
drain : 

Diameter  of  Drain.  To  Dr*in  an,.Area  not         Corresponding  Velocity  of 

Exceeding  Water  in  Dram. 

4  inches 290  square  feet.  0.30 foot. 

6        "     750       "         "  0.35     " 

8        "     1530       "        "  0.40    " 

10        "     2780       "        "  0.46    " 

12        "     4400       "        "  0.51     " 

And  for  larger  drains,  including  the  main  drains,  their  cross- 
sections  at  any  point  should  be  at  least  ^^TF  °f  tne  area  drained, 
giving  a  velocity  of  0.55  foot  per  second  with  the  rate  of 
filtration  mentioned  above. 

The  total  friction  of  the  underdrains  from  the  most  remote 
points  to  the  outlet  will  be  friction  in  the  gravel,  plus  friction  in 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


the  lateral  drains,  plus  the  friction  in  main  drain,  plus  the  veloc- 
ity head. 

I  have  calculated  in  this  way  the  friction  of  one  of  the  Ham- 
burg filters  for  the  rate  of  1,600,000  gallons  per  acre  daily  at 
which  it  is  used.  The  friction  was  calculated  for  each  section 
of  the  drains  separately,  so  that  the  friction  from  intermediate 
points  was  also  known.  Kutter's  formula  was  used  throughout 
with  n  =  0.013.  On  the  accompanying  plan  of  the  filter  I  have 


Iff] 
3 


mt  We 


r 


\          I 


V 


0:55  o< 


.19  M 


. 


n 


1 


InVJt  WelLj 


10 


20 


30 


40 


50 


60 


Meters 


0  60  100  160  200  260  Feet 

FIG.  4. — PLAN  OF  ONE  OF  THE  HAMBURG  FILTERS,  SHOWING  FRICTIONAL  RESIST- 
ANCE OF  THE  UNDERDRAINS. 

drawn  the  lines  of  equal  frictional  resistance  from  the  junction 
of  the  main  drain  with  the  last  laterals.  My  information  was 
incomplete  in  regard  to  one  or  two  points,  so  that  the  calcula- 
tion may  not  be  strictly  accurate,  but  it  is  nearly  so  and  will 
illustrate  the  principles  involved. 

The  extreme  friction  of  the  underdrains  is  11  millimeters 
=  0.036  foot. 

The  frictional  resistance  of  the  sand  39  inches  thick,  effective 
size  0.32  mm.  and  rate  1.60  million  gallons  per  acre  daily,  when 
absolutely  free  from  clogging, -is  by  the  formula,  page  21,  15 


FILTERING   MATERIALS.  39 

mm.,  or  .0490  foot,  when  the  temperature  is  50°.  Practically 
there  is  some  matter  deposited  upon  the  surface  of  the  sand 
before  filtration  starts,  and  further,  after  the  first  scraping,  there 
is  some  slight  clogging  in  the  sand  below  the  layer  removed  by 
scraping.  We  can  thus  safely  take  the  minimum  frictional 
resistance  of  the  sand  including  the  surface  layer  at  .07  foot. 
The  average  friction  of  the  underdrains  for  all  points  is  about 
.023  foot  and  the  friction  at  starting  will  be  .07  +  .023  =  .093 
foot  (including  the  friction  in  the  last  section  to  the  effluent 
well  where  the  head  is  measured,  .100  foot,  but  the  fric- 
tion beyond  the  last  lateral  does  not  affect  the  uniformity  of 
filtration).  The  actual  head  on  the  sand  close  to  the  outlet  will 

be  .093  and  the  rate  of  filtration  -23.  1.60  =  2.12.     The  actual 

.070 

head  at  the  most  remote  point  will  be  .093  —  .036  =  .057,  and  the 
rate  of  filtration  will  there  be  — —  .  160  =  1.30  million  gallons  per 

acre  daily.  The  extreme  rates  of  filtration  are  thus  2.12  and 
1.30,  instead  of  the  average  rate  of  1.60.  As  can  be  seen  from 
the  diagram,  only  very  small  areas  work  at  these  extreme 
rates,  the  great  bulk  of  the  area  working  at  rates  much  nearer 
the  average.  Actually  the  filter  is  started  at  a  rate  below  i.6or 
and  the  nearest  portion  never  filters  so  rapidly  as  2.12,  for  when 
the  rate  is  increased  to  the  standard,  the  sand  has  become  so  far 
clogged  that  the  loss  of  head  is  more  than  the  .07  foot  assumed, 
and  the  differences  in  the  rates  are  correspondingly  reduced. 
Taking  this  into  account,  it  would  not  seem  that  the  irregularities 
in  the  rate  of  filtration  are  sufficient  to  affect  seriously  the  action 
of  the  filter.  They  could  evidently  have  been  largely  reduced 
by  moderately  increasing  the  sizes  of  the  lower  ends  of  the 
underdrains,  where  most  of  the  friction  occurs  with  the  high 
velocities  (up  to  .97  foot)  which  there  result. 

The   underdrains  of   the  Warsaw  filters  were   designed   by 
Lindley  to  have  a  maximum  loss  of  head  of  only  .0164  foot  when 


4O  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

filtering  at  a  rate  of  2.57,  which  gives  a  variation  of  only  10  per 
cent  in  the  rates  with  the  minimum  loss  of  head  of  .169  foot  in 
the  entire  filter  assumed  by  him.  The  underdrains  of  the  Berlin 
filters,  according  to  my  calculations,  have  .020  to  .030  foot  friction, 
of  which  an  unusually  large  proportion  is  in  the  gravel,  owing  to 
the  excessive  distances,  in  some  cases  over  80  feet,  which  the 
gravel  is  required  to  carry  the  water.  In  this  case,  using  less  or 
finer  gravel  would  obviously  have  been  fatal,  but  the  friction  as 
well  as  the  expense  of  construction  would  be  much  reduced  by 
using  more  drains  and  less  gravel. 

The  underdrains  might  appropriately  be  made  slightly 
smaller,  with  a  deep  layer  of  fine  sand,  than  under  opposite  con- 
ditions, as  in  this  case  the  increased  friction  in  the  drains 
would  be  no  greater  in  proportion  to  the  increased  friction  in 
the  sand  itself. 

The  underdrains  of  a  majority  of  European  filters  have  water- 
tight pipes  connecting  with  them  at  intervals,  and  going  up 
through  the  sand  and  above  the  water,  where  they  are  open  to 
the  air.  These  pipes  were  intended  to  ventilate  the  underdrains 
and  allow  the  escape  of  air  when  the  filter  is  filled  with  water 
introduced  from  below.  It  may  be  said,  however,  that  in  case 
the  drains  are  surrounded  by  gravel  and  there  is  an  opportunity 
for  the  air  to  pass  from  the  top  of  the  drain  into  the  gravel,  it 
will  so  escape  without  special  provision  being  made  for  it,  and 
go  up  through  the  sand  with  the  much  larger  quantity  of  air 
in  the  upper  part  of  the  gravel  which  is  incapable  of  being  re- 
moved by  pipes  connecting  with  the  drains. 

These  ventilator  pipes  where  they  are  used  are  a  source  of 
much  trouble,  as  unfiltered  water  is  apt  to  run  down  through 
cracks  in  the  sand  beside  them,  and,  under  bad  management, 
unfiltered  water  may  even  go  down  through  the  pipes  them- 
selves. I  am  unable  to  find  that  they  are  necessary,  except  with 
underdrains  so  constructed  that  there  is  no  other  chance  for  the 
escape  of  air  from  the  tops  of  them,  or  that  they  serve  any  useful 


FILTERING  MATERIALS.  4 1 

purpose,  while  there  are  positive  objections  to  their  use.  In 
some  of  the  newer  filters  they  have  been  omitted  with  satis- 
factory results. 


DEPTH   OF  WATER  ON   THE   FILTERS. 

In  the  older  works  with  but  crude  appliances  for  regulating 
the  rate  of  filtration  and  admission  of  raw  water,  a  considerable 
depth  of  water  was  necessary  upon  the  filter  to  balance  irregu- 
larities in  the  rates  of  filtration  ;  the  filter  was  made  to  be,  to 
a  certain  extent,  its  own  storage  reservoir.  When,  however, 
appliances  of  the  character  to  be  described  in  Chapter  IV  are 
used  for  the  regulation  of  the  incoming  water,  and  with  a  steady 
rate  of  filtration,  this  provision  becomes  quite  superfluous. 

With  open  filters  a  depth  of  water  in  excess  of  the  thickness 
of  any  ice  likely  to  be  formed  is  required  to  prevent  disturb- 
ance or  freezing  of  the  sand  in  winter.  It  is  also  frequently 
urged  that  with  a  deep  water  layer  on  the  filter  the  water  does 
not  become  so  much  heated  in  summer,  but  this  point  is  not  be- 
lieved to  be  well  taken,  for  in  any  given  case  the  total  amount  of 
heat  coming  from  the  sun  to  a  given  area  is  constant,  and  the 
quantity  of  water  heated  in  the  whole  day — that  is,  the  amount 
filtered — is  constant,  and  variations  in  the  quantity  exposed  at 
one  time  will  not  affect  the  average  resulting  increase  in  tempera- 
ture. If  the  same  water  remained  upon  the  filter  without  change 
it  would  of  course  be  true  that  a  thin  layer  would  be  heated 
more  than  a  deep  one,  but  this  is  not  the  case. 

It  is  also  sometimes  recommended  that  the  depth  of  water 
should  be  sufficient  to  form  a  sediment  layer  before  filtration 
starts,  but  this  point  would  seem  to  be  of  doubtful  value,  espe- 
cially where  the  filter  is  not  allowed  to  stand  a  considerable  time 
with  the  raw  water  upon  it  before  starting  filtration. 

It  is  also  customary  to  have  a  depth  of  water  on  the  filter  in 
excess  of  the  maximum  loss  of  head,  so  that  there  can  never  be  a 


42  FILTRATION  OF  PUBLIC    WATER-SUPPLIES. 

suction  in  the  sand  just  below  the  sediment  layer.  It  may  be 
said  in  regard  to  this,  however,  that  a  suction  below  is  just  as 
effective  in  making  the  water  pass  the  sand  as  an  equal  head 
above.  At  the  Lawrence  Experiment  Station  niters  have  been 
repeatedly  used  with  a  water  depth  of  only  from  6  to  12  inches, 
with  losses  of  head  reaching  6  feet,  without  the  slightest  in- 
convenience. The  suction  only  commences  to  exist  as  the 
increasing  head  becomes  greater  than  the  depth  of  water,  and 
there  is  no  way  in  which  air  from  outside  can  get  in  to  relieve  it. 
In  these  experimental  niters  in  winter,  when  the  water  is  com- 
pletely saturated  with  air,  a  small  part  of  the  air  comes  out  of  the 
water  just  as  it  passes  the  sediment  layer  and  gets  into  reduced 
pressure,  and  this  air  prevents  the  satisfactory  operation  of  the 
filters.  But  this  is  believed  to  be  due  more  to  the  warming  and 
consequent  supersaturation  of  the  water  in  the  comparatively 
warm  places  in  which  the  filters  stand  than  to  the  lack  of  press- 
ure, and  as  not  the  slightest  trouble  is  experienced  at  other 
seasons  of  the  year,  it  may  be  questioned  whether  there  would 
be  any  disadvantage  at  any  time  in  a  corresponding  arrangement 
on  a  large  scale  where  warming  could  not  occur. 

The  depths  of  water  actually  used  in  European  filters  with 
the  full  depth  of  sand  are  usually  from  36  to  52  inches.  In  only  a 
very  few  unimportant  cases  is  less  than  the  above  used,  and  only 
a  few  of  the  older  works  use  a  greater  depth,  which  is  not  fol- 
lowed in  any  of  the  modern  plants.  As  the  sand  becomes  re- 
duced in  thickness  by  scraping,  the  depth  of  water  is  correspond- 
ingly increased  above  the  figures  given  until  the  sand  is  replaced. 
The  depth  of  water  on  the  German  covered  filters  is  quite  as 
great  as  upon  corresponding  open  filters.  Thus  the  Berlin  cov- 
ered filters  have  51,  while  the  new  open  filters  at  Hamburg  have 
only  43  inches. 


RATE   OF  FILTRATION  AND   LOSS   OF  HEAD.  43 


CHAPTER  IV. 
RATE  OF  FILTRATION  AND  LOSS  OF  HEAD. 

THE  rate  of  filtration  recommended  and  used  has  been  grad- 
ually reduced  during  the  past  thirty  years.  In  1866  Kirkwood 
found  that  12  vertical  feet  per  day,  or  3.90  million  gallons  per 
acre  daily,  was  recommended  by  the  best  engineers,  and  was 
commonly  followed  as  an  average  rate.  In  1868  the  London 
filters  averaged  a  yield  of  2.18  million  gallons*  per  acre  daily, 
including  areas  temporarily  out  of  use,  while  in  1885  the  quantity 
had  been  reduced  to  1.61.  Since  that  time  the  rate  has  appar- 
ently been  slightly  increased. 

The  Berlin  filters  at  Stralau  constructed  in  1874  were  built  to 
filter  at  a  rate  of  3.21  million  gallons  per  acre  daily.  The  first 
filters  at  Tegel  were  built  for  a  corresponding  rate,  but  have  been 
used  only  at  a  rate  of  2.57,  while  the  more  recent  filters  were  calcu- 
lated for  this  rate.  The  new  Hamburg  filters,  1892-3,  were  only 
intended  to  filter  at  a  rate  of  1.60  million  gallons  per  acre  daily. 
These  in  each  case  (except  the  London  figures)  are  the  standard 
rates  for  the  filter-beds  actually  in  service. 

In  practice  the  area  of  filters  is  larger  than  is  calculated  from 
these  figures,  as  filters  must  be  built  to  meet  maximum  instead  of 
average  daily  consumptions,  and  a  portion  of  the  filtering  area  usu- 
ally estimated  at  from  5  to  15  per  cent,  but  in  extreme  cases  reach- 
ing 50  per  cent,  is  usually  being  cleaned,  and  so  is  for  the  time  out 
of  service.  In  some  works  also  the  rate  of  filtration  on  starting  a 
filter  is  kept  lower  than  the  standard  rate  for  a  day  or  two,  or  the 
first  portion  of  the  effluent,  supposed  to  be  of  inferior  quality,  is 

*  The  American  gallon  is  used  throughout  this  book;  the  English  gallon  is  one  fifth 
larger. 


44  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

wasted,  the  amount  so  lost  reaching  in  an  extreme  case  9  to  14  per 
cent  of  the  total  quantity  of  water  filtered.*  In  many  of  the  older 
works  also,  there  is  not  storage  capacity  enough  for  filtered  water 
to  balance  the  hourly  fluctuations  in  consumption,  and  the  filters 
must  be  large  enough  to  meet  the  maximum  hourly  as  well  as 
the  maximum  daily  requirements.  For  these  reasons  the  actual 
quantity  of  water  filtered  in  a  year  is  only  from  50  to  75  per  cent 
of  what  would  be  the  case  if  the  entire  area  of  the  filters  worked 
constantly  at  the  full  rate.  A  statement  of  the  actual  yields  of  a 
number  of  filter  plants  is  given  in  Appendix  IV.  The  figures  for 
the  average  annual  yields  can  be  taken  as  quite  reliable.  The 
figures  given  for  rate,  in  many  cases,  have  little  value,  owing  to 
the  different  ways  in  which  they  are  calculated  at  different  places. 
In  addition  most  of  the  old  works  have  no  adequate  means  of 
determining  what  the  rate  at  any  particular  time  and  for  a  single 
filter  really  is,  and  statements  of  average  rates  have  only  limited 
value.  The  filters  at  Hamburg  are  not  allowed  to  filter  faster 
than  i. 60  or  those  at  Berlin  faster  than  2.57  million  gallons  per 
acre  daily,  and  adequate  means  are  provided  to  secure  this  con- 
dition. Other  German  works  aim  to  keep  within  the  latter  limit. 
Beyond  this,  unless  detailed  information  in  regard  to  methods  is: 
presented,  statements  of  rate  must  be  taken  with  some  allowance. 

EFFECT   OF   RATE   UPON  COST   OF  FILTRATION. 

The  size  of  the  filters  required,  and  consequently  the  first  cost, 
depends  upon  the  rate  of  filtration,  but  with  increasing  rates  the 
cost  is  not  reduced  in  the  same  proportion  as  the  increase  in 
'rate,  since  the  allowance  for  area  out  of  use  is  sensibly  the  same 
for  high  and  low  rates,  and  in  addition  the  operating  expenses 
depend  upon  the  quantity  filtered  and  not  upon  the  filtering 
area.  Thus,  to  supply  10  million  gallons  at  a  maximum  rate  of  2 
million  gallons  per  acre  daily  we  should  require  10  -=-  2  =  5  acres 
-{-  i  acre  reserve  for  cleaning  =  6  acres,  while  with  a  rate  twice 

*  Piefke,  Zeitsclisift  Jiir  Hygiene,  1894,   p.  177. 


RATE   OF  FILTRATION  ANDLOSSOF  HEAD.  45 


as  great,  and  with  the  same  reserve  (since  the  same  amount  of 
cleaning  must  be  done,  as  will  be  shown  below),  we  should  re- 
quire 10  -r-  4  +  i  =  3.5  acres,  or  58  per  cent  of  the  area  required 
for  the  lower  rate.  Thus  beyond  a  certain  point  increasing  the 
rate  does  not  effect  a  corresponding  reduction  in  the  first  cost. 

The  operating  cost  for  the  same  quantity  of  water  filtered 
does  not  appear  to  be  appreciably  affected  by  the  rate.  It  is 
obvious  that  at  high  rates  filters  will  became  clogged  more 
rapidly,  and  will  so  require  to  be  scraped  oftener  than  at  low 
rates,  and  it  might  naturally  be  supposed  that  the  clogging 
would  increase  more  rapidly  than  the  rates,  but  this  does  not 
seem  to  be  the  case.  At  the  Lawrence  Experiment  Station, 
under  strictly  parallel  conditions  and  with  identically  the  same 
water,  filters  running  at  various  rates  became  clogged  with  a 
rapidity  directly  proportional  to  the  rates,  so  that  the  quantities 
of  water  filtered  between  scrapings  under  any  given  conditions 
are  the  same  whether  the  rate  is  high  or  low. 

Of  the  eleven  places  (Appendix  IV)  in  Germany  filtering  river- 
waters  from  which  statistics  are  available,  four  places  with  high 
rates,  Liibeck,  Stettin,  Stuttgart,  and  Magdeburg,  yielding  3.70 
million  gallons  per  acre  daily,  filtered  on  an  average  59  million 
gallons  per  acre  between  scrapings.  Three  other  places,  Breslau, 
Altona,  and  Frankfurt,  yielding  1.85,  passed  on  an  average  55 
million  gallons  per  acre  between  scrapings,  and  four  other  places, 
Bremen,  Konigsberg,  Brunswick  and  Posen,  yielding  1.34  million 
gallons  per  acre  daily,  passed  only  40  million  gallons  per  acre 
between  scrapings.  The  works  filtering  at  the  highest  rates  thus 
filtered  more  water  in  proportion  to  the  sand  clogged  than  did 
those  filtering  more  slowly,  but  I  cannot  think  that  this  was  the 
result  of  the  rate.  It  is  more  likely  that  some  of  the  places  have 
clearer  waters  than  others,  and  that  this  both  allows  the  higher 
rate  and  causes  less  clogging  than  the  more  turbid  waters. 

A  table  of  the  estimated  relative  interest  charges  upon  the 
costs  of  constructions,  and  of  the  annual  operating  expenses  of 
filters  at  various  rates  will  be  given  in  Chapter  IX. 


46 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


EFFECT   OF   RATE   UPON   EFFICIENCY   OF   FILTRATION. 

The  effect  of  the  rate  of  filtration  upon  the  quality  of  the 
effluent  has  been  repeatedly  investigated.  The  efficiency  almost 
uniformly  decreases  rapidly  with  increasing  rate.  Frankel  and 
Piefke*  first  found  that  with  the  high  rates  the  number  of  bac- 
teria passing  some  experimental  filters  was  greatly  increased. 
Piefkef  afterward  repeated  these  experiments,  eliminating  some 
of  the  features  of  the  first  series  to  which  objection  was  made, 
and  confirmed  the  first  results.  The  results  were  so  marked 
that  Piefke  was  led  to  recommend  the  extremely  low  limit  of 
1.28  million  gallons  per  acre  daily  as  the  safe  maximum  rate  of 
filtration,  but  he  has  since  repeatedly  used  2.57  million  gallons. 

Kiimmel,J  on  the  other  hand,  in  a  somewhat  limited  series 
of  experiments,  was  unable  to  find  any  marked  connection 
between  the  rate  and  the  efficiency,  a  rate  of  2.57  giving  slightly 
better  results  than  Yates  of  either  1.28  or  5.14. 

The  admirably  executed  experiments  made  at  Zurich  in 
1886-8  upon  this  point,  which  gave  throughout  negative  results, 
have  but  little  value  in  this  connection,  owing  to  the  extremely 
low  number  of  bacteria  in  the  original  water. 

At  Lawrence  in  1892  the  following  percentages  of  bacteria 
(B.  prodigiosus]  passed  at  the  respective  rates : 


No.  of 
Filter. 

Depth. 

Effective 
Size  of 
Sand. 

Rate.     Million  gallons  per  acre  daily. 

0.5 

I.O 

i-5 

2.0 

3-o 

33A 

34A 
36A 

% 

39 
40 
42 

60 
60 
60 
60 
24 

12 
12 

12 

O.I4 
0.09 
0.20 
O.2O 
O.20 
O.2O 
O.2O 
O.20 

O.OO2 
O.OOI 

o  040 

O.OO5 

O.O2O 

O.050 
O.OIO 

0.140 

0.130 
0.  110 
0.080 
O.OQO 

o.  150 

0.050 

O.OlS 
O.OI4 

0.016 

O.3IO 
0.520 

0.550 

0.07O 
0.07O 

Avei 

'ao"e. 

O.OIO 

0.048 

0.067 

0.088 

0.356 

*  Zeitschrift  fur  Hygiene,  1891,  page  38. 

\Journalfiir  Gas-  u.   Wasserversorgung,  1891,  208  and  228. 

\  Journal  fiir  Gas-  u.   Wasserversorgung,  1893,  161. 


RATE   OF  FILTRATION  AND    LOSS   OF  HEAD.  47 

These  results  show  a  very  marked  decrease  in  efficiency  with 
increasing  rates,  the  number  of  bacteria  passing  increasing  in 
general  as  rapidly  as  the  square  of  the  rate.  The  1893  results 
also  showed  decreased  efficiency  with  high  rates,  but  the  range 
in  the  rates  under  comparable  conditions  was  less  than  in  1892, 
and  the  bacterial  differences  were  less  sharply  marked. 

While  the  average  results  at  Lawrence,  as  well  as  most  of  the 
European  experiments,  show  greatly  decreased  efficiency  with 
high  rates,  there  are  many  single  cases,  particularly  with  deep 
layers  of  not  too  coarse  sand,  where,  as  in  Kummel's  experiments, 
there  seems  to  be  little  connection  between  the  rate  and  effi- 
ciency. An  explanation  of  these  apparently  abnormal  results 
will  be  given  in  Chapter  VI. 

It  is  commonly  stated  *  that  every  water  has  its  own  special 
rate  of  filtration,  which  must  be  determined  by  local  experiments, 
and  that  this  rate  may  vary  widely  in  different  cases.  Thus  it  is 
possible  that  the  rate  of  1.60  adopted  at  Hamburg  for  the  tur- 
bid Elbe  water,  the  rate  of  2.57  used  at  Berlin,  and  about  the  same 
at  London  for  much  clearer  river-waters,  and  the  rate  of  7.50 
used  at  Zurich  for  the  almost  perfectly  clear  lake-water  are  in 
each  case  the  most  suitable  for  the  respective  waters.  In  other 
cases  however,  where  rates  much  above  2.57  are  used  for  river- 
waters,  as  at  Liibeck  and  Stettin,  there  is  a  decided  opinion  that 
these  rates  are  excessive,  and  in  these  instances  steps  are  now 
being  taken  to  so  increase  the  filtering  areas  as  to  bring  the  rates 
within  the  limit  of  2.57  million  gallons  per  acre  daily. 

From  the  trend  of  European  practice  it  would  seem  that  for 
American  river-waters  the  rate  of  filtration  should  not  exceqd 
2.57  in  place  of  the  3.90  million  gallons  per  acre  daily  recom- 
mended by  Kirkwood,  or  even  that  a  somewhat  lower  rate  might 
be  desirable  in  some  cases.  Of  course,  in  addition  to  the  area 

*  Samuelson's  translation  of  Kirkvvood's  "Filtration  of  River-waters;"  Lindley, 
Die  Nutzbarmachung  des  Flusswassers,  Journal  fiir  Gas-  u.  Wasserversorgung,  1890, 
501;  Kaiserlichen  Gesundheitsamt,  Grundsatze  fiir  die  Reinigung  von  Oberflachenwas- 
ser  durch  Sandfiltration;  Journal  fiir  Gas-  u.  Wasserversorgung,  1894,  Appendix  I. 


48  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

necessary  to  give  this  rate,  a  reserve  for  fluctuating-  rates  and  for 
cleaning  should  be  provided,  reducing  the  average  yield  to  2.00, 
1.50,  or  even  less.  In  the  case  of  water  from  clear  lakes,  ponds, 
or  storage  reservoirs,  especially  when  they  are  not  subject  to  ex- 
cessive sewage  pollution  or  to  strong  algae  growths,  it  would 
seem  that  rates  somewhat  and  perhaps  in  some  cases  very  much 
higher  (as  at  Zurich)  could  be  satisfactorily  used. 

THE   LOSS   OF   HEAD. 

The  loss  of  head  is  the  difference  between  the  heads  of  the 
waters  above  and  below  the  sand  layer,  and  represents  the  fric- 
tional  resistance  of  that  layer.  When  a  filter  is  quite  free  from 
clogging  this  frictional  resistance  is  small,  but  gradually  increases 
with  the  deposit  of  a  sediment  layer  from  the  water  filtered  until 
it  becomes  so  great  that  the  clogging  must  be  removed  by 
scraping  before  the  process  can  be  continued.  After  scraping 
the  loss  of  head  is  reduced  to,  or  nearly  to,  its  original 
amount.  With  any  given  amount  of  clogging  the  loss  of  head  is 
directly  proportional  to  the  rate  of  filtration ;  that  is,  if  a  filter 
partially  clogged,  filtering  at  a  rate  of  i.o,  has  a  frictional  resist- 
ance of  0.5  ft,  the  resistance  will  be  doubled  by  increasing  the 
rate  to  2.00  million  gallons  per  acre  daily,  provided  no  disturb- 
ance of  the  sediment  layer  is  allowed.  This  law  for  the  frictional 
resistance  of  water  in  sand  alone  also  applies  to  the  sediment  layer, 
as  I  have  found  by  repeated  tests,  although  in  so  violent  a  change 
as  that  mentioned  above,  the  utmost  care  is  required  to  make  the 
change  gradually  and  prevent  compression  or  breaking  of  the 
sediment  layer.  From  this  relation  between  the  rate  of  filtration 
and  the  loss  of  head  it  is  seen  that  the  regulation  of  either  in 
volves  the  regulation  of  the  other,  and  it  is  a  matter  of  indiffer- 
ence which  is  directly  and  which  indirectly  controlled. 

REGULATION    OF  THE    RATE   AND   LOSS   OF   HEAD    IN  THE    OLDER 

FILTERS. 

In  the  older  works,  and  in  fact  in  all  but  a  few  of  the  newest 


RATE   OF  FILTRATION  AND    LOSS   OF  HEAD.  49 

works,  the  tmderdrains  of  the  filters  connect  directly  through  a 
pipe  with  a  single  gate  with  the  pure-water  reservoir  or  pump- 
well,  which  is  so  built  that  the  water  in  it  may  rise  nearly  or 
quite  as  high  as  that  standing  upon  the  filter. 

A  typical  arrangement  of  this  sort  was  used  at  the  Stralau 
works  at  Berlin  (now  discontinued),  Fig.  5.     With  this  arrange- 


FIG.  5.— SIMPLEST  FORM  OF  REGULATION  :  STRALAU  FILTERS  AT  BERLIN. 

ment  the  rate  of  filtration  is  dependent  upon  the  height  of  water 
in  the  reservoir  or  pump-well,  and  so  upon  the  varying  con- 
sumption. When  the  water  in  the  receptacle  falls  with  increas- 
ing consumption  the  head  is  increased,  and  with  it  the  rate  of 
filtration,  while,  on  the  other  hand,  with  decreasing  draft  and 
rising  water  in  the  reservoir,  the  rate  of  filtration  decreases  and 
would  eventually  be  stopped  if  no  water  were  used.  This  very 
simple  arrangement  thus  automatically,  within  limits,  adjusts 
the  rate  of  filtration  to  the  consumption,  and  at  the  same  time 
always  gives  the  highest  possible  level  of  water  in  the  pump- 
well,  thus  also  economizing  the  coal  required  for  pumping. 

In  plants  of  this  type  the  loss  of  head  may  be  measured  by 
floats  on  little  reservoirs  built  for  that  purpose,  connected  with 
the  underdrains  ;  but  more  often  there  is  no  means  of  determining 
it,  although  the  maximum  loss  of  head  at  any  time  is  the  difference 
between  the  levels  of  the  water  on  the  filter  and  in  the  reser- 
voir, or  the  outlet  of  the  drain-pipe,  in  case  the  latter  is  above 


SO  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

the  water-line  in  the  reservoir.  The  rate  of  nitration  can  only  be 
measured  with  this  arrangement  by  shutting  off  the  incoming 
water  for  a  definite  interval,  and  observing  the  distance  that  the 
water  on  the  filter  sinks.  The  incoming  water  is  regulated  sim- 
ply by  a  gate,  which  a  workman  opens  or  closes  from  time  to 
time  to  hold  the  required  height  of  water  on  the  filter. 

The  only  possible  regulation  of  the  rate  and  loss  of  head  is 
effected  by  a  partial  closing  of  the  gate  on  the  outlet-pipe,  by 
which  the  freshly-cleaned  filters  with  nearly-closed  gates  are  kept 
from  filtering  more  rapidly  than  the  clogged  filters,  the  gates  of 
which  are  opened  wide.  Often,  however,  this  is  not  done,  and 
then  the  fresh  filters  filter  many  times  as  rapidly  as  those  which 
are  partially  clogged. 

A  majority  of  the  filters  now  in  use  are  built  more  or  less  upon 
this  plan,  including  most  of  those  in  London  and  also  the  Altona 
works,  which  had  such  a  favorable  record  with  cholera  in  1892. 

The  invention  and  application  of  methods  of  bacterial  exam- 
ination in  the  last  years  have  led  to  different  ideas  of  filtration 
from  those  which  influenced  the  construction  of  the  earlier 
plants.  As  a  result  it  is  now  regarded  as  essential  by  most 
German  engineers  *  that  each  filter  shall  be  provided  with  de- 
vices for  measuring  accurately  and  at  any  time  both  the  rate  of 
filtration  and  the  loss  of  head,  and  for  controlling  them,  and  also 
for  making  the  rate  independent  of  consumption  by  reservoirs 
for  filtered  water  large  enough  to  balance  hourly  variations 
(capacity  \  to  \  maximum  daily  quantity)  and  low  enough  so  that 
they  can  never  limit  the  rate  of  filtration  by  causing  back-water  on 
the  filters.  These  points  are  now  insisted  upon  by  the  German 
Imperial  Board  of  Health,f  and  all  new  filters  are  built  in  accord- 
ance with  them,  while  most  of  the  old  works  are  being  built  over 
to  conform  to  the  requirements. 

*  Lindley,  Journal  fur  Gas- u.   Wasserversorgung,   1890,  501  ;  Grahn,  Jotirnal  fur 
Gas-  u.   Wasserversorgung,  1890,  511  ;  Halbertsma,  Journal  fur  Gas.  u.  Wasserversor- 
, 1892,  686  ;  Piefke,  Zeitschrift  fur  Hygiene,  1894,  151  ;  and  others, 
f  Appendix  I. 


RATE   OF  FILTRATION  AND    LOSS   OF  HEAD. 


APPARATUS   FOR   REGULATING   THE   RATE  AND   LOSS   OF  HEAD. 

Many  appliances  have  been  invented  for  the  regulation  of  the 
rate  and  loss  of  head.  In  the  apparatus  designed  by  Gill  and  used 
at  both  Tegel  and  Muggel  at  Berlin  the  regulation  is  effected  by 
partially  closing  a  gate  through  which  the  effluent  passes  into  a 
chamber  in  which  the  water-level  is  practically  constant  (Fig.  6). 


III1TII1 


8  Meters 


10  15  20  25  Feet 

FIG.  6. — REGULATION  APPARATUS  AT  BERLIN  (TEGEL). 

The  rate  is  measured  by  the  height  of  water  on  the  weir  which 
serves  as  the  outlet  for  this  second  chamber  into  a  third  connect- 
ing with  the  main  reservoir,  while  the  loss  of  head  is  shown  by 
the  difference  in  height  of  floats  upon  water  in  the  first  chamber, 
representing  the  presure  in  the  underdrains,  and  upon  water  in 
connection  with  the  raw  water  on  the  filter.  From  the  respective 
heights  of  the  three  floats  the  attendant  can  at  any  time  see  the 
rate  of  filtration  and  the  loss  of  head,  and  when  a  change  is  re- 
quired it  is  effected  by  moving  the  gate. 

In  the  apparatus  designed  in  1866  by  Kirkwood  for  St.  Louis 
and  never  built  (Fig.  7)  the  loss  of  head  was  directly,  and  the  rate 
indirectly,  regulated  by  a  movable  weir,  which  was  to  have  been 
lowered  from  time  to  time  by  the  attendant  to  secure  the  re- 
quired results.  This  plan  is  especially  remarkable  as  it  meets 


52  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

the  modern  requirements  of  a  regular  rate  independent  of 
rate  of  consumption  and  of  the  water-level  in  the  reservoir,  and 
also  allows  continual  measurements  of  both  rate  (height  of  water 


AM. SANK  NOTE  CO.M.r. 


8  Meters 
25  Feet 

FIG.  7. — REGULATION  APPARATUS  AND  SECTION  OF  FILTER  RECOMMENDED  FOR 
ST.  Louis  BY  KIRKWOOD  IN  i866. 


L*      I       *       t  *  f       f      1 

"iTrniil  ...'...  1    .  .   .'   .    .  .'.  ..'...   1 

|     i     I     I     i     I     i     |     I     i    §     i  i   I     i    I  i     i  r  i     I     i     i     i     i 

o           i          A  is          • 


on  the  weir)  and  head  (difference  in  water-levels  on  filter  and  in 
effluent  chamber)  to  be  made,  and  control  of  the  same  by  the 
position  of  the  weir.  Mr.  Kirkwood  found  no  filters  in  Europe 
with  such  appliances,  and  it  was  many  years  after  his  report 
was  published  before  similar  "devices  were  used,  but  they  are 
now  regarded  as  essential. 

The  regulators  for  new  filters  at  Hamburg  (Fig.  8)  are  built 


FIG.  8.— REGULATION  APPARATUS  USED  AT  HAMBURG. 

upon  the  principle  of  Kirkwood's  device,  but  provision  is  made 
for  a  second  measurement  of  the  water  if  desired  by  the  loss  of 


RATE   OF  FILTRATION  AND    LOSS   OF  HEAD. 


53 


head  in  passing  a  submerged  orifice.  Both  the  rate  and  loss  of 
head  are  indicated  by  a  float  on  the  first  chamber  connecting 
directly  with  the  underdrain,  which  at  the  same  time  indicates 
the  head  on  a  fixed  scale,  the  zero  of  which  corresponds  to  the 
height  of  the  water  above  the  filter,  and  the  rate  upon  a  scale 
moving  with  the  weir,  the  zero  of  which  corresponds  with 
the  edge  of  the  weir.  The  water  on  the  filter  is  held  at  a  per- 
fectly constant  level. 

The  regulators  in  use  at  Worms  and  those  recently  introduced 
at  Magdeburg  act  upon  the  same  principle,  but  the  levels  of 
the  water  on  the  filters  are  allowed  to  fluctuate,  and  the  weirs 
and  in  fact,  the  whole  regulating  appliances  are  mounted  on 
big  floats  in  surrounding  chambers  of  water  connecting  with  the 
unfiltered  water  on  the  filters.  I  am  unable  to  find  any 
advantages  in  these  appliances,  and  they  are  much  more 
complicated  than  the  forms  shown  by  the  cuts. 


APPARATUS   FOR   REGULATING   THE   RATE  DIRECTLY. 

The  above-mentioned  regulators  control  directly  the  loss  of 
head,  and  only  indirectly  the  rate  of  filtration.     The  regulators 


0  5  10  15  2*)  Feet 

FIG.  9. — LINDLEY'S  REGULATION  APPARATUS  AT  WARSAW,  RUSSIA. 

at  Warsaw  were  designed  by  Lindley  to  regulate  the  rate  di- 
rectly and  make  it  independent  of  the  loss  of  head.  The  quan- 
tity of  water  flowing  away  is  regulated  by  a  float  upon  the  water 


54  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

in  the  effluent  chamber,  which  holds  the  top  of  the  telescope  out- 
let-pipe a  constant  distance  below  the  surface  and  so  secures  a 
constant  rate.  As  the  friction  of  the  filter  increases  the  float 
sinks  with  the  water  until  it  reaches  bottom,  when  the  filter  must 
be  scraped.  A  counter-weight  reduces  the  weight  on  the  float, 
and  at  the  same  time  allows  a  change  in  the  rate  when  desired. 
This  apparatus  is  automatic.  All  of  the  other  forms  described 
require  to  be  occasionally  adjusted  by  the  attendant,  but  the  at- 
tention they  require  is  very  slight,  and  watchmen  are  always  on 
duty  at  large  plants,  who  can  easily  watch  the  regulators.  The 
Warsaw  apparatus  is  reported  to  work  very  satisfactorily,  no 
trouble  being  experienced  either  by  leaking  or  sticking  of  the 
telescope-joint,  which  is  obviously  the  weakest  point  of  the 
device,  but  fortunately  a  perfectly  tight  joint  is  not  essential  to 
the  success  of  the  apparatus.  Regulators  acting  upon  the  same 
principle  have  recently  been  installed  at  Zurich,  where  they  are 
operating  successfully. 

Burton*  has  described  an  ingenious  device  designed  by  him 
for  the  filters  at  Tokyo,  Japan.  It  consists  of  a  double  acting 
valve  of  gun  metal  (similar  to  that  shown  by  Fig.  1 1),  through 
which  the  effluent  must  pass.  This  valve  is  opened  and  closed 
by  a  rod  connecting  with  a  piston  in  a  cylinder,  the  opposite 
sides  of  which  connect  with  the  effluent  pipe  above  and  below 
a  point  where  the  latter  is  partially  closed,  so  that  the  valve  is 
opened  and  closed  according  as  the  loss  of  head  in  passing  this 
obstruction  is  below  or  above  the  amount  corresponding  to  the 
desired  rate  of  filtration. 

The  use  of  the  Venturi  meter  in  connection  with  the  regula- 
tion of  filters  would  make  an  interesting  study,  and  has,  I  be- 
lieve, never  been  considered. 

*  The  Water  Supply  of  Towns.    London,  1894. 


RATE   OF  FILTRATION  AND    LOSS   OF  HEAD. 


55 


APPARATUS  FOR  REGULATING  THE  HEIGHT  OF  WATER   UPON 

FILTERS. 

It  will  be  seen  by  reference  to  the  diagrams  of  the  Berlin  and 
Hamburg  effluent  regulators  (Figs.  6  and  8)  that  their  perfect 
operation  is  dependent  upon  the  maintenance  of  a  constant  water- 
level  upon  the  filters.  The  old-fashioned  adjustment  of  the  inlet- 
gate  by  the  attendant  is  hardly  accurate  enough. 

The  first  apparatus  for  accurately  and  automatically  regulat- 
ing the  level  of  the  water  upon  the  filters  was  constructed  at 
Leeuwarden,  Holland,  by  the  engineer,  Mr.  Halbertsma,  who  has 
since  used  a  similar  device  at  other  places,  and  improved  forms 
of  which  are  now  used  at  Berlin  and  at  Hamburg. 

At  Berlin  (Muggel)  the  water-level  is  regulated  by  a  float 
upon  the  water  in  the  filter  which  opens  or  shuts  a  balanced 
double  valve  on  the  inlet-pipe  directly  beneath,  as  shown  in  Fig. 
10.  It  is  not  at  all  necessary  that  this  valve  should  shut  water- 
tight ;  it  is  only  necessary  that  it  should  prevent  the  continuous 
inflow  from  becoming  so  great  as  to  raise  the  water-level,  and 


FIG.  10. — REGULATION  OF  INFLOW  USED  AT  MUGGEL,  BERLIN. 

for  this  reason  loose,  easily-working  joints  are  employed.  The 
apparatus  is  placed  in  a  little  pit  next  to  the  side  of  the  filter, 
and  the  overflowing  water  is  prevented  from  washing  the  sand 
by  paving  the  sand  around  it  for  a  few  feet. 

At  Hamburg  the  same  result  is  obtained  by  putting  the  valve 


56 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


in  a  special  chamber  outside  of  the  filter  and  connected  with  the 
float  by  a  walking-beam  (Fig.  11). 

The  various  regulators  require  to  be  protected  from  cold  and 
ice  by  special  houses,  except  in  the  case  of  covered  filters,  where 
they  can  usually  be  arranged  with  advantage  in  the  filter  itself. 


0  5  10  15  20  Feet 

FIG.  IT. — REGULATION  OF  INFLOW  USED  AT  HAMBURG. 

In  regard  to  the  choice  of  the  form  of  regulator  for  both  the 
inlets  and  outlets  of  filters,  so  far  as  I  have  been  able  to  ascer- 
tain, each  of  the  modern  forms  described  as  in  use  performs  its 
functions  satisfactorily,  and  in  special  cases  any  of  them  could 
properly  be  selected  which  would  in  the  local  conditions  be  the 
simplest  in  construction  and  operation. 


LIMIT  TO   THE    LOSS   OF   HEAD. 

The  extent  to  which  the  loss  of  head  is  allowed  to  go  before 
filters  are  cleaned  differs  widely  in  the  different  works,  some  of 
the  newer  works  limiting  it  sharply  because  it  is  believed  that 
low  bacterial  efficiency  results  when  the  pressure  is  too  great, 
although  the  frequency  of  cleaning  and  consequently  the  cost  of 
operation  are  thereby  increased. 

At  Darlington,  England,  I  believe  as  a  result  of  the  German 
theories,  the  loss  of  head  is  limited  to  about  18  inches  by  a 
masonry  weir  built  within  the  last  few  years.  At  Berlin,  both  at 


RATE   OF  FILTRATION  AND    LOSS   OF  HEAD.  $? 

Tegel  and  Miiggel,  the  limit  is  24  inches,  while  at  the  new  Ham- 
burg works  28  inches  are  allowed.  At  Stralau  in  1893  an  effort 
was  made  to  not  exceed  a  limit  of  40  inches,  but  previously  heads 
up  to  60  inches  were  used,  which  corresponds  with  the  56  inches 
used  at  Altona  ;  and,  in  the  other  old  works,  while  exact  informa- 
tion is  not  easily  obtained  because  of  imperfect  records,  I  am  con- 
vinced that  heads  of  60  or  even  80  inches  are  not  uncommon. 
At  the  Lawrence  Experiment  Station  heads  of  70  inches  have 
generally  been  used,  although  some  filters  have  been  limited  to 
36  and  24  inches. 

In  1866  Kirkwood  became  convinced  that  the  loss  of  head 
should  not  go  much  above  30  inches,  first,  because  high  heads 
would,  by  bringing  extra  weight  upon  the  sand,  make  it  too 
compact,  and,  second,  because  when  the  pressure  became  too 
great  the  sediment  layer  on  the  surface  of  the  sand,  in  which 
most  of  the  loss  of  head  occurs,  would  no  longer  be  able  to 
support  the  weight  and,  becoming  broken,  would  allow  the 
water  to  pour  through  the  comparatively  large  resulting  open- 
ings at  greatly  increased  rates  and  with  reduced  efficiency. 

In  regard  to  the  first  point,  a  straight,  even  pressure  many 
times  that  of  the  water  on  the  filter  is  incapable  of  compressing 
the  sand.  It  is  much  more  the  effect  of  the  boots  of  the  work- 
men when  scraping  that  makes  the  sand  compact.  I  have  found 
sand  in  natural  banks  at  Lawrence  70  or  80  feet  below  the  sur- 
face, where  it  had  been  subjected  to  corresponding  pressure  for 
thousands  of  years,  to  be  quite  as  porous  as  when  packed  in 
water  in  experimental  filters  in  the  usual  way. 

The  second  reason  mentioned,  or,  as  I  may  call  it,  the  break- 
ing-through theory,  is  very  generally  if  not  universally  accepted 
by  German  engineers,  and  this  is  the  reason  for  the  low  limit 
commonly  adopted  by  them. 

A  careful  study  of  the  results  at  Lawrence  fails  to  show  the 
slightest  deterioration  of  the  effluents  up  to  the  limit  used,  72 
inches.  Thus  in  1892,  taking  only  the  results  of  the  continuous 


58  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

filters  of  full  height  (Nos.  33A,  34A,  36A,  and  37),  we  find  that 
for  the  three  days  before  scraping,  when  the  head  was  nearly  72 
inches,  the  averge  number  of  bacteria  in  the  effluents  was  31  per 
cc.,  while  for  the  three  days  after  scraping,  with  very  low  heads, 
the  number  was  47.  The  corresponding  numbers  of  B.  prodigio- 
sus* were  i.i  and  2.7.  This  shows  better  work  with  the  highest 
heads,  but  is  open  to  the  objection  that  the  period  just  after  scrap- 
ing, owing  to  the  disturbance  of  the  surface,  is  commonly  sup- 
posed to  be  a  period  of  low  efficiency. 

To  avoid  this  criticism  in  calculating  the  corresponding 
results  for  1893,  the  numbers  of  the  bacteria  for  the  intermediate 
days  which  could  not  have  been  influenced  either  by  scraping 
or  by  excessive  head  are  put  side  by  side  with  the  others.  Tak- 
ing these  results  as  before  for  continuous  filters  72  inches  high, 
and  excluding  those  with  extremely  fine  sands  and  a  filter  which 
was  only  in  operation  a  short  time  toward  the  end  of  the  year, 
we  obtain  the  following  results : 

Water  B. 

Bacteria  Prodigiosus 

per  cc.  per  cc. 

Average  ist  day  after  scraping,  low  heads 79  6.1 

2d    "        "            "            "        "     44  4-1 

"      3d    "        "  "  "        "    .-45  3-6 

Intermediate  days,  medium  heads 59  4.5 

Second  from  last  day,  heads  of  nearly  72  inches 66  2.7 

Next  to  the  last  day,       "       "      "        "        "      56  3.2 

Last  day,                           "        "      "       "        "      . .  83  2.5 

These  figures  show  a  very  slight  increase  of  the  water  bac- 
teria in  the  effluent  as  the  head  approaches  the  limit,  but  no 
such  increase  as  might  be  expected  from  a  breaking  through  of 
the  sediment  layer,  and  the  B.  prodigiosus  which  is  believed  to 
better  indicate  the  removal  of  the  bacteria  of  the  original  water, 

*  A  special  species  of  bacteria  artificially  added  to  secure  more  precise  information 
n  regard  to  the  passage  of  germs  through  the  filter. 


RATE   OF  FILTRATION  AND    LOSS   OF  HEAD.  59 

actually  shows  a  decrease,  the  last  day  being  the  best  day  of  the 
whole  period. 

The  Lawrence  results,  then,  uniformly  and  clearly  point  to  a 
conclusion  directly  opposite  to  the  commonly  accepted  view, 
and  I  have  thus  been  led  to  examine  somewhat  closely  the 
grounds  upon  which  the  breaking-through  theory  rests. 

The  two  works  which  have  perhaps  contributed  most  to  the 
theories  of  filtration  are  the  Stralau  and  Altona  works.  After 
examining  the  available  records  of  these  works,  I  am  quite  con- 
vinced that  at  these  places  there  has  been,  at  times  at  least,  de- 
creased efficiency  with  high  heads.  For  the  Stralau  works 
this  is  well  shown  by  Piefke's  plates  in  the  Zeitschrift  fur  Hygiene, 
1894,  after  page  188.  In  both  of  these  works,  however,  the  ap- 
paratus (or  lack  of  apparatus)  for  regulating  the  rate  is  that  shown 
by  Fig.  5,  page  49,  and  the  rate  of  filtration  is  thus  dependent 
upon  the  rate  of  consumption  and  the  height  of  water  in  the 
reservoir.  At  the  Stralau  works,  at  the  time  covered  by  the 
above-mentioned  diagrams,  the  daily  quantity  of  water  filtered 
was  27  times  the  capacity  of  the  reservoir,  and  the  rate  of  filtra- 
tion must  consequently  have  adapted  itself  to  the  hourly  con- 
sumptions. The  data  which  formed  the  basis  of  Kirkwood's 
conclusions  are  not  given  in  detail,  but  it  is  quite  safe  to  assume 
that  they  were  obtained  from  filters  regulated  as  those  at  Altona 
and  Stralau  are  regulated,  and  what  is  said  in  regard  to  the 
latter  will  apply  equally  to  his  results. 

Piefke*  shows  that  among  the  separate  filters  at  Stralau,  all 
connected  with  the  same  pure-water  reservoir,  those  connected 
through  the  shorter  pipes  gave  poorer  effluents  than  the  more 
remote  filters,  and  he  attributes  the  difference  to  the  frictional 
resistance  of  the  connecting  pipes,  which  helped  to  prevent  ex- 
cessive rates  in  the  filters  farthest  away  when  the  water  in  the 
reservoir  became  low,  and  thus  the  fluctuations  in  the  rates  in 
these  filters  were  less  than  in  those  close  to  the  reservoir.  He 

*  Zeitschrift  fiir  Hygiene,  1894,  p.  173. 


60  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

does  not,  however,  notice,  in  speaking  of  the  filters  in  which 
the  decreased  efficiencies  with  high  heads  were  specially  marked, 
that  they  follow  in  nearly  the  same  order,  and  that  of  the  four 
open  filters  mentioned  three  were  near  the  reservoir  and  only 
one  was  separated  by  a  comparatively  long  pipe,  indicating  that 
the  deterioration  with  high  heads  was  only  noticeable,  or  at 
least  was  much  more  conspicuous,  in  those  filters  where  the  rates 
fluctuated  most  violently. 

It  requires  no  elaborate  calculation  to  show  that  of  two  filters 
connected  with  the  same  pure-water  reservoir,  as  shown  by  Fig. 
5,  with  only  simple  gates  on  the  connecting  pipes,  one  of  them 
clean  and  throttled  by  a  nearly  closed  gate,  so  that  the  normal 
pressure  behind  the  gate  is  above  the  highest  level  of  water 
in  the  reservoir,  and  the  other  clogged  so  that  the  normal 
pressure  of  the  water  in  the  drain  is  considerably  below  the 
highest  level  of  the  water  in  the  reservoir,  the  latter  will  suffer 
much  the  more  severe  shocks  with  fluctuating  water-levels ;  and 
the  fact  being  admitted  that  fluctuating  levels  are  unfavorable, 
we  must  go  farther  and  conclude  that  the  detrimental  action 
will  increase  with  increasing  loss  of  head.  I  am  inclined  to 
think  that  this  theory  is  adequate  to  explain  the  Stralau  and 
Altona  results  without  resource  to  the  breaking-through  theory. 

While  the  above  does  not  at  all  prove  the  breaking-through 
theory  to  be  false,  it  explains  the  results  upon  which  it  rests  in 
another  way,  and  can  hardly  fail  to  throw  so  much  doubt  upon 
it  as  to  make  us  refuse  to  allow  its  application  to  those  works 
where  a  regular  rate  of  filtration  is  maintained  regardless  of 
variations  in  the  consumption,  until  proof  is  furnished  that  it  is 
applicable  to  them. 

I  have  been  totally  unable  to  find  satisfactory  European  re- 
sults in  regard  to  this  point.  The  English  works  can  furnish 
nothing,  both  on  account  of  the  lack  of  regulating  appliances 
and  because  the  monthly  bacterial  examinations  are  inadequate 
for  a  discussion  of  hourly  or  daily  changes.  The  results  from 


RATE   OF  FILTRATION  AND   LOSS   OF  HEAD.  6 1 

the  older  Continental  works  are  also  excluded  for  one  or  the 
other,  or  more  often  for  both,  of  the  above  reasons.  The  Ham- 
burg, Tegel,  and  Miiggel  results,  so  far  as  they  go,  show  no  de- 
terioration with  increased  heads,  but  the  heads  are  limited  to  24 
or  28  inches  by  the  construction  of  the  niters,  and  the  results 
thus  entirely  fail  to  show  what  would  be  obtained  with  heads 
more  than  twice  as  high. 

I  am  thus  forced  to  conclude  that  there  is  no  adequate  evi- 
dence of  inferior  efficiency  with  high  heads  in  filters  where  the 
rates  are  independent  of  the  water-level  in  the  pure-water 
reservoir,  the  only  results  directly  to  the  point — the  Lawrence 
results  mentioned  above — indicating  that  the  full  efficiency  is 
maintained  with  heads  reaching  at  least  72  inches. 

The  principal  reason  for  desiring  to  allow  a  considerable  loss 
of  head  is  an  economical  one  ;  the  period  will  then  be  length- 
ened, while  the  frequency  of  scraping  and  the  volume  of  sand  to 
be  washed  and  replaced  will  be  correspondingly  reduced.  There 
may  be  other  advantages  in  long  periods,  such  as  less  trouble 
with  scraping  and  better  work  in  cold  winter  weather,  but  the 
cost  is  the  most  important  consideration. 

It  is  the  prevalent  idea  among  the  German  engineers  that 
the  loss  of  head  after  reaching  24  to  30  inches  would  increase 
very  rapidly,  so  that  the  quantity  of  water  filtered,  in  case  a 
much  higher  head  was  allowed,  would  not  be  materially  in- 
creased. No  careful  investigations,  however,  have  been  made, 
and  indeed  they  are  hardly  possible  with  existing  arrangements, 
as  in  the  older  filters  the  loss  of  head  fluctuates  with  varying 
rates  of  filtration  in  such  a  way  that  only  results  of  very  doubt- 
ful value  can  be  obtained,  and  in  the  newer  works  the  loss  of 
head  is  too  closely  limited,  and  the  curves  which  can  be  drawn 
by  extrapolation  are  evidently  no  safe  indications  of  what  would 
actually  happen  if  the  process  was  carried  farther. 

On  the  other  hand,  I  was  told  by  the  attendant  at  Darling- 
ton, England,  that  since  the  building  of  the  weir  a  few  years  ago, 


62  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

which  now  limits  the  loss  of  head  to  about  18  inches  instead  of 
the  5  feet  or  more  formerly  used,  the  quantity  of  sand  to  be  re- 
moved has  been  three  times  as  great  as  formerly.  No  records 
are  kept,  and  this  can  only  be  given  as  the  general  impression  of 
the  man  who  superintends  the  work. 

At  Lawrence  the  average  quatities  of  water  filtered  between 
scrapings  with  sand  of  an  effective  size  of  0.20  mm.  have  been  as 
follows : 

Maximum  Loss  of  Million  Gallons  per  Acre  filtered 

Head.  between  Scrapings. 

1892.  1893.         Average. 

70  inches 58     88     73 

34   "  32     22     27 

22   "  17     16     16 

With  sand  of  an  effective  size  of  0.29  mm.  the  results  were  : 

1893. 
70  inches 70 

22         " 29 

These  results  indicate  a  great  increase  in  the  quantity  of  water 
filtered  between  scrapings  with  increasing  heads,  the  figures 
being  nearly  proportional  to  the  maximum  heads  used  in  the 
respective  cases.  It  is,  of  course,  quite  possible  that  the  results 
would  differ  in  different  places  with  the  character  of  the  raw 
water  and  of  the  filtering  material. 

The  depth  of  sand  to  be  removed  by  scraping  at  one  time  is, 
within  limits,  practically  independent  of  the  quantity  of  dirt 
which  it  has  accumulated,  and  any  lengthening  of  the  period 
means  a  corresponding  reduction  in  the  quantity  of  sand  to  be 
removed,  washed  and  replaced  and  consequently  an  important 
reduction  in  the  operating  cost,  as  well  as  a  reduction  in  the 
area  of  filters  out  of  use  while  being  cleaned,  and  so,  in  the 
capital  cost. 

Among  the  minor  objections  to  an  increased  loss  of  head  are 
the  greater  head  against  which  the  water  must  be  pumped,  and 


RATE   OF  FILTRATION  AND   LOSS   OF  HEAD.  63 

the  possible  increased  difficulty  of  filling  filters  with  filtered 
water  from  below  after  scraping,  but  these  would  hardly  have 
much  weight  against  the  economy  indicated  by  the  Lawrence 
experiments  for  the  higher  heads. 

High  heads  will  also  drive  an  increased  quantity  of  water 
through  any  cracks  or  passages  in  the  filter.  Such  leaks  have 
at  last  been  found  to  be  the  cause  of  the  inferior  work  of  the 
covered  filters  at  Stralau,  the  water  going  down  unfiltered  in 
certain  corners,  especially  at  high  heads  ;  but  with  careful  con- 
struction there  should  be  no  cracks,  and  with  the  aid  of  bac- 
teriology to  find  the  possible  leaks  this  ought  not  to  be  a  valid 
objection. 

In  conclusion  :  the  trend  of  opinion  is  strongly  in  favor  of 
limiting  the  loss  of  head  to  about  24  to  30  inches  as  was  suggested 
by  Kirkwood,  but  I  am  forced  to  conclude  that  there  is  reason 
to  believe  that  equally  good  results  can  be  obtained  with  lower 
operating  expenses  by  allowing  higher  heads  to  be  used,  at  least 
in  the  case  of  filters  with  modern  regulating  appliances,  and, 
I  would  suggest  that  filters  should  be  built  so  as  not  to  exclude 
the  use  of  moderately  high  heads,  and  that  the  limit  to  be 
permanently  used  should  be  determined  by  actual  tests  of 
efficiency  and  length  of  period  with  various  losses  of  head  after 
starting  the  works. 


64  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


CHAPTER  V. 
CLEANING  FILTERS. 

WHEN  a  filter  has  become  so  far  clogged  that  it  will  no 
longer  pass  a  satisfactory  quantity  of  water  with  the  allowable 
head  it  must  be  cleaned  by  scraping  off  and  removing  the  upper 
layer  of  dirty  sand. 

To  do  this  without  unnecessary  loss  of  time  the  unfiltered 
water  standing  upon  the  filter  is  removed  by  a  drain  above  the 
sand  provided  for  that  purpose.  The  water  in  the  sand  must 
then  be  lowered  below  the  surface  of  the  sand  by  drawing  water 
from  the  underdrains  until  the  sand  is  firm  enough  to  bear  the 
weight  of  the  workmen.  By  the  time  that  this  is  accomplished 
the  last  water  on  the  surface  should  have  soaked  away,  and  the 
filter  is  ready  to  be  scraped.  This  is  done  by  workmen  with 
wide,  sharp  shovels,  and  the  sand  removed  is  taken  to  the  sand- 
washing  apparatus  to  be  washed  and  used  again.  Special  pains 
are  given  to  securing  rapid  and  cheap  transportation  of  the  sand. 
In  some  cases  it  is  wheeled  out  of  the  filter  on  an  inclined  plane 
to  the  washer.  In  other  cases  a  movable  crane  is  provided 
which  lifts  the  sand  in  special  receptacles  and  allows  it  to  fall 
into  cars  on  a  tram-line  on  which  the  crane  also  moves.  The 
cars  as  filled  are  run  to  the  washer  and  also  serve  to  bring 
back  the  washed  sand.  When  the  dirty  sand  has  been  removed, 
the  surface  of  the  sand  is  carefully  smoothed  and  raked.  This  is 
especially  necessary  to  remove  the  effects  of  the  workmen's 
boots. 

It  is  customary  in  the  most  carefully  managed  works  to  fill 
the  sand  with  filtered  water  from  below,  introduced  through  the 
underdrains.  In  case  the  ordinary  level  of  the  water  in  the 


CLEANING   FILTERS.  65 

pure-water  canal  is  higher  than  the  surface  of  the  sand  in  the 
filters,  this  is  accomplished  by  simply  opening  a  gate  provided 
for  the  purpose,  which  allows  the  water  to  pass  around  the 
regulating  apparatus.  Otherwise  filters  can  be  filled  from  a 
special  pipe  taking  its  water  from  any  filter  which  at  that  time 
can  deliver  its  effluent  high  enough  for  that  purpose.  The 
quantity  of  water  required  for  filling  the  sand  from  below  is 
ordinarily  but  a  fraction  of  one  per  cent  of  the  quantity  filtered. 

Formerly,  instead  of  filling  from  below,  after  cleaning,  the 
raw  water  was  brought  directly  onto  the  surface  of  the  filter. 
This  was  said  to  only  imperfectly  fill  the  sand-pores,  which 
still  contained  much  air.  If,  however,  the  water  is  not  brought 
on  too  rapidly  it  will  sink  into  the  sand  near  the  point  where  it 
is  applied,  pass  laterally  through  the  sand  or  underlying  gravel 
to  other  parts  of  the  filter,  and  then  rise,  so  that  even  in  this' 
case  all  but  a  little  of  the  filter  will  be  really  filled  from  below. 
This  is,  however,  open  to  the  objection  that  however  slowly  the 
water  is  introduced,  the  sand  which  absorbs  it  around  the  inlet 
filters  it  at  a  very  high  rate  and  presumably  imperfectly,  so  that 
the  water  in  the  underdrains  at  the  start  will  be  poor  quality 
and  the  sand  around  the  inlet  will  be  unduly  clogged.  The 
practice  of  filling  from  below  is  therefore  well  founded. 

As  soon  as  the  surface  of  the  sand  is  covered  with  the  water 
from  below,  raw  water  is  introduced  from  above,  filling  the  filter 
to  the  standard  height,  care  being  taken  at  first  that  no  currents 
are  produced  which  might  wash  the  surface  of  the  sand.  It  has 
been  recommended  by  Piefke  and  others  that  this  water  should 
be  allowed  to  stand  for  a  time  up  to  twenty-four  hours  before 
starting  the  filtration,  to  allow  the  formation  of  a  sediment  layer, 
and  in  some  places,  especially  at  Berlin  and  the  works  of  some 
of  the  London  companies,  this  is  done ;  but  varying  importance 
is  attached  to  the  procedure,  and  it  is  invariably  omitted,  so  far 
as  I  can  learn,  when  the  demand  for  water  is  heavy. 

The  depth  of  sand  removed  by  scraping  must  at  least  equal 


-66 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


the  depth  of  the  discolored  layer,  but  there  is  no  sharp  dividing- 
line,  the  impurities  gradually  decreasing  from  the  surface  down- 
ward. Fig.  12  shows  the  relative  number  of  bacteria  found  in 
the  sand  at  various  depths  in  one  of  the  Lawrence  experimental 
niters,  and  is  a  representative  result,  although  the  actual  num- 
bers vary  at  different  times.  In  general  it  may  be  said  that  the 
bulk  of  the  sediment  is  retained  in  the  upper  quarter  inch,  but 
it  is  desirable  to  remove  also  the  less  dirty  sand  below  and,  in 
fact,  it  is  apparently  impossible  with  the  method  of  scraping  in 
use  to  remove  so  thin  a  layer  as  one  fourth  inch.  Practically 


Bacteria  per  Gram. 
100000 500000 


1000000 


Layerremoved  by  Scraping 


FIG.  12. — DIAGRAM  SHOWING  ACCUMULATION  OF  BACTERIA  NEAR  THE  SURFACE 

OF  THE  SAND. 


the  depth  to  which  sand  is  removed  is  stated  to  be  from  0.40 
to  i. 20  inch.  Exact  statistics  are  not  easily  obtained,  but  I  think 
that  2  centimeters  or  0.79  inch  may  be  safely  taken  as  about 
the  average  depth  usually  removed  in  European  filters,  and  it 
is  this  depth  which  is  indicated  on  Fig.  12. 

At  the  Lawrence  Experiment  Station,  the  depth  removed 
is  often  much  less  than  this,  and  depends  upon  the  size  of  grain 
•of  the  sand  employed,  the  coarser  sands  requiring  to  be  more 
deeply  scraped  than  the  finer  ones.  The  method  of  scraping, 
however,  which  allows  the  removal  of  very  thin  sand  layers,  is 


CLEANING   FILTERS.  6/ 

only  possible  because  of  the  small  size  of  the  filters,  and  as  it 
is  incapable  of  application  on  a  large  scale,  the  depths  thus 
removed  are  only  interesting  as  showing  the  results  which 
might  be  obtained  in  practice  with  a  more  perfect  method  of 
scraping. 

The  replacing  of  the  washed  sand  is  usually  delayed  until 
the  filter  has  been  scraped  quite  a  number  of  times — commonly 
for  a  year.  The  last  scraping  before  refilling  is  much  deeper 
than  usual,  because  the  sand  below  the  depth  of  the  ordinary 
scraping  is  somewhat  dirty,  and  might  cause  trouble  if  left  below 
the  clean  sand. 

In  England  it  is  the  usual  if  not  the  universal  practice  to 
replace  the  washed  sand  at  the  bottom  between  the  old  sand 
and  the  gravel.  This  is  done  by  digging  up  the  entire  filter 
in  sections  about  six  feet  wide.  The  old  sand  in  the  first  section 
is  removed  clear  down  to  the  gravel,  and  the  depth  of  washed 
sand  which  is  to  be  replaced  is  put  in  its  place.  The  old  sand 
from  the  next  six-foot  section  is  then  shovelled  upon  the  first 
section  of  clean  sand,  and  its  place  is  in  turn  filled  with  fresh 
sand.  With  this  practice  the  workmen's  boots  are  likely  to 
disturb  the  gravel  each  year,  necessitating  a  thicker  layer  of  the 
upper  and  finest  grade  than  would  otherwise  be  required. 

In  Germany  this  is  also  sometimes  done,  but  more  frequently 
the  upper  layer  of  slightly  clogged  sand  below  the  regular  scrap- 
ing is  removed  as  far  as  the  slightest  discoloration  can  be  seen, 
perhaps  6  inches  deep.  The  sand  below  is  loosened  for  another  6 
inches  and  allowed  to  stand  dry,  if  possible,  for  some  days  ;  after- 
wards the  washed  sand  is  brought  on  and  placed  above.  The 
washed  sand  is  never  replaced  without  some  such  treatment,  be- 
cause the  slightly  clogged  sand  below  the  layer  removed  would 
act  as  if  finer  than  the  freshly  washed  sand,*  and  there  would  be 
a  tendency  to  sub-surface  clogging. 

*  Report  Mass.  State  Board  of  Health  for  1891,  p.  438  ;    1892,  page  409. 


68  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

FREQUENCY   OF   SCRAPING. 

The  frequency  of  scraping  depends  upon  the  character  of 
the  raw  water,  the  thoroughness  of  the  preliminary  sedimenta- 
tion, the  grain-size  of  the  filter  sand,  the  rate  of  filtration,  and 
the  maximum  loss  of  head  allowed.  With  suitable  conditions 
the  period  between  scrapings  should  never  be  less  than  one 
week,  and  will  but  rarely  exceed  two  months.  Under  excep- 
tional conditions,  however,  periods  have  been  recorded  as  low 
as  one  day  and  as  high  as  one  hundred  and  ten  days.  Periods 
of  less  than  a  week's  duration  are  almost  conclusive  evidence 
that  something  is  radically  wrong,  and  the  periods  of  one  day 
mentioned  were  actually  accompanied  by  very  inadequate  filtra- 
tion. In  1892  the  average  periods  at  the  German  works  varied 
from  9.5  days  at  Stettin  (with  an  excessive  rate)  to  40  days  at 
Brunswick,  the  average  of  all  being  25  days.* 

The  quantity  of  water  per  acre  filtered  between  scrapings 
forms  the  most  convenient  basis  for  calculation.  The  effect  of 
rate  (page  45)»  loss  of  head  (page  61),  and  size  of  sand  grain  (page 
28)  have  already  been  discussed,  and  it  will  suffice  to  say  here 
that  the  total  quantity  filtered  between  scrapings  is  apparently 
independent  of  the  rate  of  filtration,  but  varies  with  the  maxi- 
mum loss  of  head  and  with  the  grain-size  of  the  sand,  and 
apparently  nearly  in  proportion  to  them.  Eleven  German  filter- 
works  in  1892,  drawing  their  waters  from  rivers,  filtered  on  an 
average  51  million  gallons  of  water  per  acre  between  scrapings, 
the  single  results  ranging  from  28  at  Bremen  to  71  at  Stuttgart, 
while  Zurich,  drawing  its  water  from  a  lake  which  is  but  very 
rarely  turbid,  filtered  260  million  gallons  per  acre  between  scrap- 
ings. Unfortunately,  the  quantities  at  Berlin,  where  (in  1892 
two  thirds  and  now  all)  the  water  is  drawn  from  comparatively 
large  ponds  on  the  rivers,  are  not  available  for  comparison. 

At  London,  in  1884,  the  average  quantities  of  water  filtered 

*  Appendix  IV. 


CLEANING   FILTERS.  69 

between  scrapings  varied  from  43  to  136  million  gallons  per 
acre  with  the  different  companies,  averaging  85,  and  in  1892  the 
quantities  ranged  from  73  to  157,  averaging  90  million  gallons 
per  acre.  The  greater  quantity  filtered  at  London  may  be  due 
to  the  greater  sizes  of  the  sedimentation-basins,  which  for  all  the 
companies  together  hold  a  nine  days'  supply  at  London  against 
probably  less  than  one  day's  supply  for  the  German  works. 

There  is  little  information  available  in  regard  to  the  fre- 
quency of  scraping  with  water  drawn  from  impounding  reser- 
voirs. In  some  experiments  made  by  Mr.  FitzGerald  at  the 
Chestnut  Hill  reservoir,  Boston,  the  results  of  which  are  as  yet 
unpublished,  a  filter  with  sand  of  an  effective  size  of  only  .09 
mm.  averaged  58  million  gallons  per  acre  between  scrapings 
for  nine  periods,  the  rate  of  filtration  being  1.50  million  gallons 
per  acre  daily,  while  another  filter,  with  sand  of  an  effec- 
tive size  of  .18  mm.,  passed  an  average  of  93  million  gallons 
per  acre  for  ten  periods  at  the  same  rate.  These  experiments 
extended  through  all  seasons  of  the  year,  and  taking  into  ac- 
count the  comparative  fineness  of  the  sands  they  show  rather 
high  quantities  of  water  filtered  between  scrapings. 

The  quantity  of  water  filtered  between  scrapings  is  usually 
greatest  in  winter,  owing  to  the  smaller  quantity  of  sediment  in 
the  raw  water  at  this  season,  and  is  lowest  in  times  of  flood, 
regardless  of  season.  In  summer  the  quantity  is  often  reduced 
to  a  very  low  figure  in  waters  supporting  algas  growths,  es- 
pecially when  the  filters  are  not  covered.  Thus  at  Stralau  in 
1893  during  the  algas  period  the  quantity  was  reduced  to  14 
million  gallons  per  acre  for  open  filters,*  but  this  was  quite 
exceptional,  the  much-polluted,  though  comparatively  clear, 
Spree  water  furnishing  unusually  favorable  conditions  for  the 
algae. 

*Piefke,  Zeitschrift  fur  Hygiene,  1894,  p,  177. 


70  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

QUANTITY   OF  SAND    TO   BE   REMOVED. 

In  regard  to  the  quantity  of  sand  to  be  removed  and  washed, 
if  we  take  the  average  result  given  above  for  the  German  works 
filtering  river-waters  of  51,000,000  gallons  per  acre  filtered  be- 
tween scrapings,  and  the  depth  of  sand  removed  at  two  centi- 
meters or  0.79  inch,  we  find  that  one  volume  of  sand  is  required  for 
every  2375  volumes  of  water  filtered,  or  2.10  cubic  yards  per 
million  gallons.  At  Bremen,  the  highest  average  result,  the 
quantity  would  be  3.80  yards,  and  at  Stralau  during  the  algae 
season  7.70  yards.  At  Zurich,  on  the  other  hand,  the  quantity  is 
only  0.41  yard,  and  at  London,  with  87,000,000  gallons  per  acre 
filtered  between  scrapings,  the  quantity  of  sand  washed  would 
be  1.24  yards  per  million  gallons  ;  assuming  always  that  the  layer 
removed  is  0.79  inch  thick. 

These  estimates  are  for  the  regular  scrapings  only,  and  do 
not  include  the  annual  deeper  scraping  before  replacing  the 
sand,  which  would  increase  them  by  about  one  third. 

WASTING  THE  EFFLUENTS  AFTER  SCRAPING. 

It  has  already  been  stated  that  an  important  part  of  the  fil- 
tration takes  place  in  the  sediment  layer  deposited  on  top  of  the 
sand  from  the  water.  When  this  layer  is  removed  by  scraping 
its  influence  is  temporarily  removed,  and  reduced  efficiency  oi 
filtration  may  result.  The  significance  of  this  reduced  efficiency 
became  apparent  when  the  bacteria  in  the  water  were  studied  ii 
their  relations  to  disease,  and  Piefke  suggested  *  that  the  first 
effluent  after  scraping  should  be  rejected  for  one  day  after 
ordinary  scrapings  and  for  one  week  after  replacing  the  sand. 
In  a  more  recent  paper  f  he  reduces  these  estimates  to  th< 
first  million  gallons  of  water  per  acre  filtered  after  scraping 

*  Journal  fur  Gas-  und  Wasserversorgung,  1887,  p.  595. 
f  Zeitschrift  fur  Hygiene,  1894,   p.  172. 


CLEANING   FILTERS.  7 1 

for  open  and  twice  as  great  a  quantity  for  covered  filters, 
and  to  six  days  after  replacing  the  sand,  which  last  he  estimates 
will  occur  only  once  a  year.  Taking  the  quantity  of  water 
filtered  between  scrapings  at  13.9  million  gallons  per  acre,  the 
quantity  observed  at  Stralau  in  the  summer  of  1893,  he  finds 
that  it  is  necessary  to  waste  9  per  cent  of  the  total  quantity  of 
effluent  from  open  and  13.8  per  cent  of  that  from  covered  filters. 

The  eleven  German  water-works*  filtering  river-waters, 
however,  filtered  on  an  average  51.0  instead  of  13.9  million  gal- 
lons per  acre  between  scrapings,  and  applying  Piefke's  figures  to 
them  the  quantities  of  water  to  be  wasted  would  be  only  about 
one  fourth  of  his  estimates  for  Stralau. 

The  rules  of  the  Imperial  Board  of  Health  f  require  that 
every  German  filter  shall  be  so  constructed  "  that  when  an  in- 
ferior effluent  results  it  can  be  disconnected  from  the  pure- 
water  pipes  and  the  filtrate  allowed  to  be  wasted."  The  drain- 
pipe for  removing  the  rejected  water  should  be  connected 
below  the  apparatus  for  regulating  the  rate  and  loss  of  head,  so 
that  the  filter  can  be  operated  exactly  as  usual,  and  the  effluent 
can  be  turned  back  to  the  pure-water  pipes  without  stopping  or 
changing  the  rate.  The  works  at  Berlin  and  at  Hamburg  con- 
form to  this  requirment,  and  most  of  the  older  German  works 
have  been  or  are  being  built  over  to  make  them  do  so. 

In  regard  to  the  extent  of  deterioration  after  scraping, 
Piefke's  experiments  have  always  shown  much  larger  numbers 
of  bacteria  both  of  the  ordinary  forms  and  of  special  applied 
forms  on  the  first  day  after  scraping,  the  numbers  frequently 
being  many  times  as  high  as  at  other  times. 

At  the  Lawrence  Experiment  Station  it  was  found  in  1892 
that  on  an  average  the  number  of  water  bacteria  was  increased  by 
70  per  cent  (continuous  filters  only)  for  the  three  days  following 
scraping,  while  B.prodigiosus  when  applied  was  increased  140  per 


Appendix  IV.  f  Appendix  I. 

OF  THK 

UNIVESITY 


72  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

cent,  the  increase  being  most  marked  where  the  depth  of  sand 
was  least,  and  with  the  highest  rate  of  filtration. 

The  same  tendency  was  found  in  1893,  when  the  increase  ii 
the  water  bacteria  on  the  first  day  after  scraping  was  only  i< 
per  cent  and  B.  prodigiosus  64  per  cent,  but  for  a  portion  of  th< 
year  the  difference  was  greater,  averaging  132  and  262  per  cenl 
respectively.  These  differences  are  much  less  than  those  n 
corded  by  Piefke,  and  with  the  high  efficiencies  regularly  ol 
tained  at  Lawrence  they  would  hardly  justify  the  expensivi 
practice  of  wasting  the  effluent. 

The  reduction  in  efficiency  following  scraping  is  much  1< 
at  low   rates,  and  if  a  filter  is  started  at  much   less  than   n 
normal  rate  after  scraping,  and  then  gradually  increased  to  thi 
standard  after  the  sediment  layer  is  formed,  the  poor  work  will 
be  largely  avoided.      Practically  this  is  done  at  Berlin  and  al 
Hamburg.    The  filters  are  started  at  a  fourth  or  less  of  the  usu< 
rates  and  are  gradually  increased,  as  past  experience  with  ba< 
terial  results  has  shown  it  can  be  safely  done,  and  the  effluent  is 
then  even  at  first  so  well  purified  that  it  need  not  be  wasted. 

Practically  in  building  new  filters  the  provision  of  a  suitable 
connection  for  wasting  the  effluents  into  the  drain  which 
necessary  for  emptying  them  involves  no  serious  expense  an< 
should  be  provided,  but  it  may  be  questioned  how  often  ii 
should  be  used  for  wasting  the  effluents.  If  the  raw  water  is 
bad  that  a  good  effluent  cannot  be  obtained  by  careful  manipub 
tion  even  just  after  scraping,  the  course  of  the  Berlin  authoriti< 
in  closing  the  Stralau  works  and  seeking  a  less  polluted  suppl; 
would  seem  to  be  the  only  really  safe  procedure. 


SAND-WASHING. 

The  sand-washing  apparatus  is  an  important  part  of  most 
European  filtering  plants.  It  seldom  happens  that  a  natural  san< 
can  be  found  clean  enough  and  sufficiently  free  from  fine  par- 


CLEANING   FILTERS. 


73 


tides,  although  such  a  sand  was  found  and  used  for  the  Lawrence 
filter.  Most  of  the  sand  in  use  for  filtration  in  Europe  was 
originally  washed.  In  the  operation  of  the  filters  also,  sand- 
washing  is  used  for  the  dirty  sand,  which  can  then  be  used  over 
and  over  at  a  much  lower  cost  than  would  be  the  case  if  fresh 
sand  was  used  for  refilling.  The  methods  used  for  washing 
sand  at  the  different  works  present  a  great  variety  both  in  their 
details  and  in  the  underlying  principles.  Formerly  boxes  with 
double  perforated  bottoms  in  which  the  sand  was  placed  and 
stirred  by  a  man  as  water  from  below  rose  through  them,  and 
other  similar  arrangements  were  commonly  used,  but  they  are 
at  present  only  retained,  so  far  as  I  know,  in  some  of  the  smaller 
English  works.  The  cleansing  obtained  is  apparently  consider- 
ably less  thorough  than  with  some  of  the  modern  devices. 


FIG.  13. — HOSE-WASHING  FOR  DIRTY  SAND. 

Hose-washing  is  used  in  London  by  the  Southwark  and  Vaux- 
hall,  Lambeth  and  Chelsea  companies,  and  also  at  Antwerp. 
For  this  a  platform  is  constructed  about  15  feet  long  by  8  feet 
wide,  with  a  pitch  lengthwise  of  6  to  8  inches  (Fig.  13).  The 


74  FILTRA7ION  OF  PUBLIC  WATER-SUPPLIES. 

platform  is  surrounded  by  a  wall  rising  from  one  foot  at  the  bottom 
to  three  feet  high  at  the  top,  except  the  lower  end,  which  is  closed 
by  a  removable  plank  weir  5  or  6  inches  high.  From  two  to 
four  cubic  yards  of  the  sand  are  placed  upon  this  platform  and  a 
stream  of  water  from  a  hose  with  a  £  or  |-inch  nozzle  is  played 
upon  it,  moving  it  about  from  place  to  place.  The  sand  itself 
is  always  kept  toward  the  upper  end  of  the  platform,  while  the 
water  with  the  dirt  removed  flows  down  into  the  pond  made  by 
the  weir,  where  the  sand  settles  out  and  the  dirt  overflows  with 
the  water.  When  the  water  comes  off  clear,  which  is  usually 
after  an  hour  or  a  little  less,  the  weir  is  removed,  and,  after 
draining,  the  sand  is  removed.  These  arrangements  are  built  in 
pairs  so  that  the  hose  can  be  used  in  one  while  the  sand  is  being 
changed  in  the  other.  They  are  usually  built  of  brick  laid  in 
cement,  but  plank  and  iron  are  also  used.  The  corners  are 
sometimes  carried  out  square  as  in  the  figure,  but  are  more  often 
rounded.  The  washing  is  apparently  fairly  well  done. 

In  Germany  the  so-called  "  drum  "  washing-machine,  drawings 
of  which  have  been  several  times  published,*  has  come  to  be 
almost  universally  used.  It  consists  of  a  large  revolving  cylin- 
der, on  the  bottom  of  the  inside  of  which  the  sand  is  slowly 
pushed  up  toward  the  higher  end  by  endless  screw-blades  at- 
tached to  the  cylinder,  while  water  is  freely  played  upon  it  all 
the  way.  The  machine  requires  a  special  house  for  its  accommo- 
dation and  from  2  to  4  horse-power  for  its  operation.  It  washes 
from  2.5  to  4  yards  of  sand  per  hour  most  thoroughly,  with  a 
consumption  of  from  1 1  to  14  times  as  large  a  volume  of  water. 
The  apparatus  is  not  patented  or  made  for  sale,  but  full  plans 
can  be  easily  secured. 

A  machine  made  by  Samuel  Pegg  &  Sons,  Leicester,  Eng., 
pushes  the  sand  up  a  slight  incline  down  which  water  flows.  It 
is  very  heavy  and  requires  power  to  operate  it.  The  patent  has 

*  Glaser's  Annalen,  1886,  p.  48  ;  Zeit.  f.  Hygiene,  1889,  p.  128. 


CLEANING   FILTERS.  75 

expired.  A  machine  much  like  it  but  lighter  and  more  conven- 
ient and  moved  by  water-power  derived  from  the  water  used 
for  washing  instead  of  steam-power  is  used  at  Zurich  with 
good  results. 

o 

Iii  Green  way's  machine  the  sand  is  forced  by  a  screw  through 
a  long  narrow  cylinder  in  which  there  is  a  current  of  water  in 
the  opposite  direction.  The  power  required  is  furnished  by  a 
water-motor,  as  with  the  machine  at  Zurich.  The  apparatus  is 
mounted  on  wheels  and  is  portable  ;  it  has  an  appliance  for 
piling  up  the  washed  sand  or  loading  it  onto  cars.  It  is 
patented  and  is  manufactured  by  James  Gibb  £  Co.,  London. 

Several  of  the  London  water  companies  are  now  using 
ejector  washers,  and  such  an  apparatus  has  been  placed  by  the 
side  of  the  "  drum  "  washers  at  Hamburg.  This  apparatus  was 
made  by  Korting  Brothers  in  Hannover,  and  combines  the 
ejectors  long  made  by  that  firm  with  hoppers  from  designs 
by  Mr.  Bryan,  engineer  of  the  East  London  Water  Company. 
An  apparatus  differing  from  this  only  in  the  shape  of  the 
ejectors  and  some  minor  details  has  been  patented  in  England, 
and  is  for  sale  by  Messrs.  Hunter,  Frazer  &  Goodman,  Bow, 
London. 

Both  of  these  forms  consist  of  a  series  of  conical  hoppers, 
irom  the  bottom  of  each  of  which  the  sand  and  water  are  forced 
into  the  top  of  the  next  by  means  of  ejectors,  the  excess  of  dirty 
water  overflowing  from  the  top  of  each  hopper.  The  apparatus 
is  compact  and  not  likely  to  get  out  of  order,  but  is  not  portable. 
It  can  be  easily  arranged  to  take  the  sand  at  the  level  of  the 
ground,  or  even  lower  if  desired,  and  deliver  it  washed  at  some 
little  elevation,  thus  minimizing  hand-labor.  The  washing  is 
regular  and  thorough.  The  objection  most  frequently  raised 
against  its  use  is  the  quantity  of  water  required,  but  at 
Hamburg  I  was  informed  that  the  volume  of  water  required 
was  only  about  15  times  that  of  the  sand,  while  almost  as  much 
(13-14  volumes)  were  required  for  the  "  drum  "  washers,  and 


76  FILTRATION   OF  PUBLIC  WATER-SUPPLIES. 

the  saving  in  power  much  more  than  offset  the  extra  cost  for 
water. 

In  addition  to  the  above  processes  of  sand-washing,  Piefke's 
method  of  cleaning  without  scraping*  might  be  mentioned,  al- 
though as  yet  it  has  hardly  passed  the  experimental  stage,  and 
has  only  been  used  on  extremely  small  filters.  The  process  con- 
sists of  stirring  the  surface  sand  of  the  filter  with  "  waltzers  " 
while  a  thin  sheet  of  water  rapidly  flows  over  the  surface.  This 
arrangement  necessitates  a  special  construction  of  the  filters,  pro- 
viding for  rapidly  removing  the  unfiltered  water  from  the  sur- 
face, and  for  producing  a  regular  and  rapid  movement  of  a  thin 
sheet  of  water  over  the  surface.  In  the  little  filters  now  in  use, 
one  of  which  I  saw  in  a  brewery  in  Berlin,  the  cleaning  is  rapidly, 
cheaply,  and  apparently  well  done. 

In  washing  dirty  sand  it  is  obvious  that  any  small  sand-grains 
will  be  removed  with  the  dirt,  and  in  washing  new  sand  the  main 
object  is  to  remove  the  grains  below  a  certain  size.  It  is  also 
apparent  that  the  sizes  of  grains  which  will  and  those  which  will 
not  be  removed  are  dependent  upon  the  mechanical  arrange- 
ments of  the  washer,  as,  for  example,  with  the  ejectors,  upon  the 
sizes  of  the  hoppers,  and  the  quantity  of  water  passing  through 
them,  and  care  should  be  taken  to  make  them  correspond  with 
the  size  of  grain  selected  for  the  filter  sand.  This  can  only  be 
done  by  experiment,  as  no  results  are  available  on  this  point. 

In  some  places  filtered  water  is  used  for  sand-washing,  al- 
though this  seems  quite  unnecessary,  as  ordinary  river-water 
answers  very  well.  It  is,  however,  often  cheaper,  especially  in 
small  works,  to  use  the  filtered  water  from  the  mains  rather  than 
provide  a  separate  supply  for  the  washers. 

The  quantity  of  water  required  for  washing  may  be  esti- 
mated at  15  times  the  volume  of  the  sand  and  the  sand  as  0.04 
per  cent  of  the  volume  of  the  water  filtered  (page  70),  so  that 

*  Vierteljahresschrift  fiir  offentliche  Gesundheitspflege,  1891,  p.  59. 


CLEANING   FILTERS.  77 

0.6  per  cent  of  the  total  quantity  of  water  filtered  will  be  re- 
quired for  sand-washing. 

The  cost  of  sand- washing  in  Germany  with  the  "  drum  " 
washers  is  said  to  be  from  14  to  20  cents  per  cubic  yard,  in- 
cluding labor,  power,  and  water.  In  America  the  water  would 
cost  no  more,  but  the  labor  would  be  perhaps  twice  as  dear. 
With  an  ejector  apparatus  I  should  estimate  the  cost  of 
washing  dirty  sand  as  follows :  The  sand  would  be  brought  and 
dumped  near  to  the  washer,  and  one  man  could  easily  feed  it  in, 
as  no  lifting  is  required.  Two  men  would  probably  be  re- 
quired to  shovel  the  washed  sand  into  barrows  or  carts  with 
the  present  arrangements,  but  I  think  with  a  little  ingenuity 
this  handling  could  be  made  easier. 

ESTIMATED   COST   OF   OPERATING   EJECTOR  WASHERS  9   HOURS. 

Wages  of  3  men  at  $2.00 $6.00 

110,000  gals,  water  (15  times  the  volume  of  sand) 
at  0.05  a  thousand  gals 5.50 


Total  cost  of  washing  36  cubic  yards $11.50 

or  32  cents  a  cubic  yard. 


The  cost  of  washing  new  sand  might  be  somewhat  less.  The 
other  costs  of  cleaning  filters,  scraping,  transporting,  and  re- 
placing the  sand  are  much  greater  than  the  washing  itself. 
Lindley  states  that  at  Warsaw  29  days'  labor  of  10  hours  for  one 
man  are  required  to  scrape  an  acre  of  filter  surface,  and  four 
times  as  much  for  the  annual  deep  scraping,  digging  up,  and 
replacing  the  sand.  The  first  expense  occurs  in  general 
monthly,  and  the  second  only  once  a  year.  At  other  places 
where  I  have  secured  corresponding  data  the  figures  range  from 
19  to  40  days'  labor  to  scrape  one  acre,  and  average  about  the 
same  as  Lindley  estimates. 

Under  some  conditions  sand-washing  does  not  pay,  and  in 


/8  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

still  others  it  is  almost  impossible.  No  apparatus  has  yet  been 
devised  which  will  wash  the  dirt  out  of  the  fine  dune-sands  used 
in  Holland  without  washing  a  large  part  of  the  sand  itself  away, 
and  in  these  works  fresh  sand,  which  is  available  in  unlimited 
quantities  and  close  to  the  works,  is  always  used.  At  Breslau 
the  dirty  sand  is  sold  for  building  purposes  for  one  third  of  the 
price  paid  for  new  sand  dredged  from  the  river,  delivered  at  the 
works,  and  no  sand  is  ever  washed.  Budapest,  Warsaw,  and 
Rotterdam  also  use  fresh  river-sand  without  washing,  except  a 
very  crude  washing  to  remove  clay  at  Budapest. 


THEORY  AND   EFFICIENCY  OF  CONTINUOUS  FILTRATION.     79 


CHAPTER  VI. 
THEORY  AND  EFFICIENCY  OF  CONTINUOUS  FILTRATION. 

THE  first  filters  for  a  public  water-supply  were  built  by  James 
Simpson,  engineer  of  the  Chelsea  Water  Company  at  London 
in  1839.  They  were  apparently  intended  to  remove  dirt  from 
the  water  in  imitation  of  natural  processes,  and  without  any  very 
clear  conception  of  either  the  exact  extent  of  purification  or  the 
way  in  which  it  was  to  be  accomplished.  The  removal  of  tur- 
bid;ty  was  the  most  obvious  result,  and  a  clear  effluent  was  the 
single  test  of  the  efficiency  of  filtration,  as  it  remains  the  legal 
criterion  of  the  work  of  the  London  filters  even  to-day,  notwith- 
standing the  discovery  and  use  of  other  and  more  delicate  tests. 
The  invention  and  use  of  methods  for  determining  the  organic 
matters  in  water  by  Wanklyn  and  Frankland,  about  1870,  led  to 
the  discovery  that  the  proportion  of  organic  matters  removed  by 
filtration  was  disappointingly  low,  and  as,  at  the  time,  and  for 
many  years  afterward,  an  exaggerated  importance  was  given  to 
the  mere  quantities  of  organic  matters  in  water,  it  was  con- 
cluded that  filtration  had  only  a  limited  influence  upon  the 
healthfulness  of  the  filtered  water,  and  that  practically  as  much 
care  must  be  given  to  securing  an  unpolluted  water  as  would 
be  the  case  if  it  were  delivered  direct  without  filtration.  This 
theory,  although  not  confirmed  by  more  recent  investigation,  un- 
doubtedly has  had  a  good  influence  upon  the  English  works  by 
causing  the  selection  of  raw  waters  free  from  excessive  pollu- 
tions, and,  in  cases  like  the  London  supplies,  drawn  from  the 
Thames  and  the  Lea,  in  stimulating  a  most  jealous  care  of  the 
watersheds  and  the  purification  of  sewage  by  the  towns  upon 
them. 


8O  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

It  was  only  after  the  discovery  of  the  bacteria  in  water  and 
their  relations  to  health  that  the  hygenic  significance  of  filtration 
commenced  to  be  really  understood.  Investigations  of  the  bac- 
teria in  the  waters  before  and  after  filtration  were  carried  out  at 
Berlin  by  Plagge  and  Proskower,  at  London  by  Dr.  Percy 
Frankland,  and  also  at  Zurich,  Altona,  and  on  a  smaller  scale 
at  other  places.  These  investigations  showed  that  the  bacteria 
were  mainly  removed  by  filtration,  the  numbers  in  the  effluents 
rarely  exceeding  two  or  three  per  cent  of  those  in  the  raw  water. 
This  gave  a  new  aspect  to  the  problem. 

It  was  further  observed,  especially  at  Berlin  and  Zurich,  that 
the  numbers  of  bacteria  in  effluents  were  apparently  quite  inde- 
pendent of  the  numbers  in  the  raw  water,  and  the  theory  was 
formed  that  all  of  the  bacteria  were  stopped  by  the  filters,  and 
that  those  found  in  the  effluents  were  the  result  of  contamination 
from  the  air  and  of  growths  in  the  underdrains.  The  logical 
conclusion  from  this  theory  was  that  filtered  water  was  quite 
suitable  for  drinking  regardless  of  the  pollution  of  its  source. 

It  was,  however,  found  that  the  numbers  of  bacteria  in  the 
effluents  were  higher  immediately  after  scraping  than  at  other 
times,  and  it  was  concluded  that  before  the  formation  of  the 
sediment  layer  some  bacteria  were  able  to  pass  the  sand,  and  it 
was  therefore  recommended  that  the  first  water  filtered  after 
scraping  should  be  rejected. 

Piefke  at  Berlin  gave  the  subject  careful  study,  and  came  to 
the  conclusion  that  it  was  almost  entirely  the  sediment  layer 
which  stopped  the  bacteria,  and  that  the  bacteria  themselves 
in  the  sediment  layer  formed  a  slimy  mass  which  completely 
intercepted  those  in  the  passing  water.  When  this  layer  was 
removed  by  scraping,  the  action  was  stopped  until  a  new  crop 
of  bacteria  had  accumulated.  In  support  of  this  idea  he  stated 
that  he  had  taken  ordinary  good  filter-sand  and  killed  the  bac- 
teria in  it- by  heating  it,  and  that  on  passing  water  through,  no 
purification  was  effected — in  fact,  the  effluent  contained  more 


THEORY  AND   EFFICIENCY  OF  CONTINUOUS  FILTRATION.     8 1 

bacteria  than  the  raw  water.  After  a  little,  bacteria  established 
themselves  in  the  sand,  and  then  the  usual  purification  was  ob- 
tained. Piefke  concluded  that  the  action  of  the  filter  was  a 
•biological  one ;  that  simple  straining  was"  quite  inadequate  to 
produce  the  results  obtained  ;  that  the  action  of  the  filter  was 
mainly  confined  to  the  sediment  layer,  and  that  the  depth  of  sand 
beyond  the  slight  depth  necessary  for  the  support  of  this  layer 
had  no  appreciable  influence  upon  the  results.  The  effect  of 
this  theory  is  still  seen  in  the  shallow  sand  layers  used  at  Berlin 
and  some  other  German  works,  although  at  London  the  tendency 
is  rather  toward  thicker  sand  layers. 

Piefke's  deductions,  however,  are  not  entirely  supported  by 
his  data  as  we  understand  them  in  the  light  of  more  recent  in- 
vestigation. The  experiment  with  sterilized  sand  has  been  re- 
peatedly tried  at  the  Lawrence  Experiment  Station  with  results 
which  quite  agree  with  Piefke's,  but  it  has  also  been  found  that 
the  high  numbers,  often  many  times  as  high  as  in  the  raw  watery 
do  not  represent  bacteria  which  pass  in  the  ordinary  course  of 
filtration,  but  instead  enormous  growths  of  bacteria  throughout 
the  sand  supported  by  the  cooked  organic  matter  in  it.  It  has 
been  repeatedly  found  that  ordinary  sand  quite  incapable  of  sup- 
porting bacterial  growths,  after  heating  to  a  temperature  capable 
of  killing  the  bacteria  will  afterwards  furnish  the  food  for  most 
extraordinary  numbers.  A  filter  of  such  sand  may  stop  the  bac- 
teria of  the  passing  water  quite  as  effectually  as  any  other  filter, 
but  if  so,  the  fact  cannot  be  determined  without  recourse  to 
special  methods,  on  account  of  the  enormous  numbers  of  bacteria 
in  the  sand,  a  small  part  of  which  are  carried  forward  by  the 
passing  water,  and  completely  mask  the  normal  action  of  the 
filter. 

The  theory  that  all  or  practically  all  of  the  bacteria  are  inter- 
cepted by  the  sediment  layer,  and  that  those  in  the  effluent  are 
the  result  of  growths  in  the  sand  or  underdrains,  received  two 
hard  blows  in  1889  and  1891,  when  mild  epidemics  of  typhoid  fever 


$2  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

followed  unusually  high  numbers  of  bacteria  in  the  effluents  at 
Altona  and  at  Stralau  in  Berlin,  with  good  evidence  in  each  case 
that  the  fever  was  directly  due  to  the  water.  Both  of  these  cases 
came  during,  and  as  the  result  of,  severe  winter  weather  with, 
open  niters  and  under  conditions  which  are  now  recognized  as 
extremely  unfavorable  for  good  nitration. 

As  a  result  of  the  first  of  these  epidemics  a  series  of  experi- 
ments were  made  at  Stralau  by  Frankel  and  Piefke  in  1890. 
Small  niters  were  constructed,  and  water  passed  exactly  as  in  the 
ordinary  niters.  Bacteria  of  special  kinds  not  existing  in  the 
raw  water  or  effluents  were  then  applied,  and  the  presence  of  a 
very  small  fraction  of  them  in  the  effluents  demonstrated  beyond 
a  doubt  that  they  had  passed  through  the  filters  under  the 
ordinary  conditions  of  filtration.  These  experiments  were  after- 
wards repeated  by  Piefke  alone  under  somewhat  different  con- 
ditions with  similar  results.  The  numbers  of  bacteria  passing, 
although  large  enough  to  establish  the  point  that  some  do  pass, 
were  nevertheless  in  general  but  a  small  fraction  of  one  per  cent 
of  the  many  thousands  applied. 

This  method  of  testing  the  efficiency  of  filters  had  already 
been  used  quite  independently  by  Prof.  Sedgwick  at  the  Law- 
rence Experiment  Station  in  connection  with  the  purification  of 
sewage,  and  has  since  been  extensively  used  there  for  experi- 
ments with  water-filtration. 

Kiimmel  also  found  at  Altona  that  while  in  the  regular  samples 
for  bacterial  examination,  all  taken  at  the  same  time  in  the  day, 
there  was  no  apparent  connection  between  the  numbers  of  bac- 
teria in  the  raw  water  and  effluents,  by  taking  samples  at  frequent 
intervals  throughout  the  twenty-four  hours,  as  has  been  done  in 
a  more  recent  series  of  experiments,  and  allowing  for  the  time 
required  for  the  water  to  pass  the  filters,  a  well-marked  connec- 
tion was  found  to  exist  between  the  numbers  of  bacteria  in  the 
raw  water  and  in  the  effluents. 

The  subject  has  more  recently  been  studied  in  much  detail  at 


THEORY  AND   EFFICIENCY  OF  CONTINUOUS  FILTRATION.     83 

the  Lawrence  Experiment  Station,  and  it  now  appears  that  the 
bacteria  in  the  effluent  from  a  filter  are  from  two  sources: 
directly  from  the  filtered  water,  and  from  the  lower  layers  of  the 
filter  and  underdrains.  Thus  we  may  say  : 

Bacteria  in  effluent  =  Bacteria  from  underdrains  +  — X  bacteria 

100 

in  raw  water, 
where  a  is  the  per  cent  of  bacteria  actually  passing  the  filter. 

Both  of  these  terms  depend  upon  a  whole  series  of  complex 
and  but  imperfectly  understood  conditions.  In  general  the  bac- 
teria from  the  underdrains  are  low  in  cold  winter  weather,  often 
almost  nil,  while  at  Lawrence  with  water  temperatures  of  70  to 
75  degrees,  and  over,  in  July  and  August,  the  numbers  from  this 
source  may  reach  200  or  300,  but  for  the  other  ten  months  of  the 
year  rarely  exceed  50  under  normal  conditions.  In  summer  espe- 
cially it  seems  to  be  greater  at  low  than  at  high  rates  of  filtration 


.2.5 


RATE  OF  FILTRATION  -MILLION  GALLONS  PER  .ACRE,  DAILY. 

FIG.  14. — SHOWING  BACTERIA  SUPPOSED   TO   COME   THROUGH    FILTERS  AND  FROM 

THE  UNDERDRAINS. 

(although  a  high  rate  for  a  short  time  only  increases  it),  and  so 
varies  in  the  opposite  way  from  the  numbers  actually  passing 
the  filters.  This  subject  is  by  no  means  clearly  understood ;  it 
is  difficult,  almost  impossible,  to  separate  the  numbers  of  bacteria 
into  the  two  parts — those  which  come  directly  through  and  may 
be  dangerous,  and  those  which  have  other  origins  and  are  harmless. 
The  sketch,  Fig.  14,  is  drawn  to  represent  my  idea  of  the  way 
they  may  be  divided.  It  has  no  statistical  basis  whatever.  The 
light  unshaded  section  shows  the  percentage  number  of  bacteria 


84  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

which  I  conceive  to  be  coming  through  a  filter  under  given  con- 
ditions at  various  rates  of  filtration,  while  the  shaded  section 
above  represents  the  bacteria  from  other  sources,  and  the  upper 
line  represents  the  sum  of  the  two,  or  the  total  number  of  bac- 
teria in  the  effluent.  The  relative  importance  of  the  two  parts 
would  probably  vary  widely  with  various  conditions.  With  the 
conditions  indicated  by  the  sketch  the  number  of  bacteria  in  the 
effluent  is  almost  constant :  for  a  variation  of  only  from  1.4  to  2.5 
per  cent  of  the  number  applied  for  the  whole  range  is  not  a  wide 
fluctuation  for  bacterial  results,  but  the  number  in  the  lower  and 
dangerous  section  is  always  rapidly  increasing  with  increasing 
rate. 

This  theory  of  filtration  accounts  for  many  otherwise  per- 
plexing facts.  The  conclusion  reached  at  Zurich  and  elsewhere 
that  the  efficiency  of  filtration  is  independent  of  rate  may  be  ex- 
plained in  this  way.  This  is  especially  probable  at  Zurich,  where 
the  number  of  bacteria  in  the  raw  water  was  only  about  200, 
and  an  extremely  large  proportion  relatively  would  have  to  pass 
to  make  a  well-marked  impression  upon  the  total  number  in  the 
effluent. 

These  underdrain  bacteria  are,  so  far  as  we  know,  entirely 
harmless ;  we  are  only  interested  in  them  to  determine  how  far 
they  are  capable  of  decreasing  the  apparent  efficiency  of  filtra- 
tion below  the  actual  efficiency,  or  the  per  cent  of  bacteria  really 
removed  by  the  filter. 

This  efficiency  is  dependent  upon  a  large  number  of  con- 
ditions many  of  which  have  already  been  discussed  in  connec- 
tion with  grain-size  of  filter  sand,  underdrains,  rate  of  filtration, 
loss  of  head,  etc.,  and  a  mere  reference  to  them  here  will  suffice. 
Perhaps  the  most  important  single  condition  is  the  rate,  the 
numbers  of  bacteria  passing  increase  rapidly  with  it.  Next,  fine 
sand  and  in  moderately  deep  layers  tends  to  give  high  efficiency. 
The  influence  of  the  loss  of  head,  often  mentioned,  is  not  shown 
to  be  important  by  the  Lawrence  results,  nor  can  I  find 


THEORY  AND   EFFICIENCY  OF  CONTINUOUS  FILTRATION.     85 

satisfactory  European  results  in  support  of  it.  Uniformity  in  the 
rate  of  nitration  on  all  parts  of  the  filtering  area  and  a  constant 
rate  throughout  the  twenty -four  hours  are  regarded  as  essential 
conditions  for  the  best  results.  Severe  winter  weather  has  in- 
directly, by  disturbing  the  regular  action  of  open  filters,  an  in- 
jurious influence,  and  has  been  the  cause  of  most  of  the  cases 
where  filtered  waters  have  been  known  to  injure  the  health  of 
those  who  have  drunk  them.  This  action  is  excluded  in  filters 
covered  with  masonry  arches  and  soil,  and  such  construction  is 
apparently  necessary  for  the  best  results  in  places  subject  to  cold 
winters. 

The  efficiency  of  filtration  under  various  conditions  has  been 
studied  by  a  most  elaborate  series  of  experiments  at  Lawrence 
with  small  filters  to  which  water  has  been  applied  containing  a 
bacterium  (B.  prodigiosus)  which  does  not  occur  naturally  in  this 
country  and  is  not  capable  of  growing  in  the  filter,  so  that  the 
results  should  represent  only  the  bacteria  coming  through  the 
filter  and  not  include  any  additions  from  the  underdrains.  These 
results,  which  have  been  published  in  full  in  the  reports  of  the 
Massachusetts  State  Board  of  Health,  especially  for  the  years 
1892  and  1893,  show  that  the  number  of  bacteria  passing  increases 
rapidly  with  increasing  rate,  and  slowly  with  decreasing  sand 
thickness  and  increased  size  of  sand-grain. 

Assuming  that  the  number  of  bacteria  passing  is  expressed 

by  the  formula 

i    (rate)'  X  effective  size  of  sand 
Per  cent  bacteria  passing  =  - 


2  l/thickness  of  the  sand  in  inches 
where  the  rate  is  expressed  in  million  gallons  per  acre  daily,  and 
calculating  by  it  the  numbers  of  bacteria  for  the  seventy-three 
months  for  which  satisfactory  data  are  available  from  1 1  filters 
in  1892  and  1893,  we  find  that 
In  14  cases  the  numbers  observed  were  4  to  9  times  as  great  as 

the  calculated  numbers ; 
In  6  cases  they  were  2  to  3  times  as  great ; 


86  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

In  35  cases   they   were  between   \  and   2   times  the  calculated 

numbers. 

In  17  cases  they  were  £  to  \  of  them. 
In  1 1  cases  they  were  less  than  \  the  calculated  numbers. 

The  agreement  is  only  moderately  good,  and  in  fact  no  such 
formula  could  be  expected  to  give  more  than  very  rough  approxi- 
mations, because  it  does  not  take  into  consideration  the  numerous 
other  elements,  such  as  uniformity  and  regularity  of  filtration, 
the  influence  of  scraping,  the  character  of  the  sediment  in  the 
raw  water,  etc.,  which  are  known  to  affect  the  results.  Perhaps 
the  most  marked  general  difference  is  the  tendency  of  new  or 
freshly-filled  filters  to  give  higher,  and  of  old  and  well-com- 
pacted filters  to  give  lower,  results  than  those  indicated  by  the 
formula. 

Comparing  this  formula  with  Piefke's  results  given  in  his 
"  Neue  Ermittelungen  "  *  the  formula  gives  in  the  first  series 
(0.34  mm.  sand,  0.50  m.  thick,  and  rate  100  mm.  per  hour),  0.25 
per  cent  passing,  while  the  average  number  of  B.  violations 
reported,  excluding  the  first  day  of  decreased  efficiency  after 
scraping,  was  0.26  per  cent.  In  the  second  series,  with  half  as 
high  a  rate  the  numbers  checked  exactly  the  calculated  0.06  per 
cent. 

In  other  experiments^  however,  in  1893,  when  the  calculated 
per  cent  was  also  0.25,  only  0.03,  0.04,  and  0.07  per  cent  were 
observed  in  the  effluents. 

Comparing  the  results  from  the  actual  filters,  (which  numbers 
also  include  the  bacteria  from  the  underdrains  and  should  there- 
fore be  somewhat  higher)  with  the  numbers  calculated  as  pass- 
ing through,  I  find  that  for  the  46  days,  Aug.  20  to  Oct.  4, 
1893,  for  which  detailed  results  of  the  Stralau  works  are  given 
by  Piefke,  the  average  calculated  number  passing  is  0.20  per 


*  Journal  fur  Gas-  und  Wasserversorgungt  1891,  108. 
f  Zeitschrift  fur  Hygiene,  1894,  182. 


THEORY  AND   EFFICIENCY   OF  CONTINUOUS  FILTRATION.      87 


cent,  while  twice  as  many  were  observed  in  the  effluents ;  al- 
though three  of  the  filters  gave  better  effluents  than  the  other 
eight,  and  the  numbers  from  them  approximated  closely  the 
calculated  numbers.  If  we  calculate  the  percentages  of  bacteria 
passing  a  number  of  filters,  using  the  maximum  rate  of  nitration 
allowed  for  the  German  filters  where  this  is  accurately  deter- 
mined,  and  for  the  English  filters  taking  the  maximum  rate  at 
one  and  one-half  times  the  rate  obtained  by  dividing  the  daily 
quantity  by  the  area  of  filters  actually  in  use,  we  obtain : 


\ 

Average 
Depth  of 
Sand, 
Inches. 

Effective 
Size  of 
Sand- 
grain. 

Maximum 
Rate  ot 
Filtration. 

Per  cent 
Bac  teria 
passing 
i     rid 

*  \'sand 

32 

O    31 

I    60 

o  07 

Altona  ....                  

28 

O   34 

2    57 

O    21 

Berlin,  Stralau  

2O 

O  34 

2    57 

0    25 

Miig-o-el 

2O 

O   34 

2    57 

O   25 

Tegel         

2O 

O   37 

2    57 

O   27 

London,  Southwark  &  Vauxhall  

36 

O   34 

2   8l 

O   22 

West  Middlesex  

3Q 

O   37 

2   8l 

o  23 

CA 

O  36 

327 

o  26 

30 

O  4O 

3   27 

O    3Q 

'        Lambeth  

36 

o  36 

3   75 

O  42 

Middlesborough                  ..       

20 

O  42 

5   85 

I    58 

26 

O    35 

7    CQ 

I    QO 

The  numbers  actually  observed  are  in  every  case  higher  than 
the  calculated  per  cents  passing,  as  indeed  they  should  be  on 
account  of  those  coming  from  the  underdrains,  accidental  con- 
tamination of  the  samples,  etc. 

It  may  be  said  that  filtration  as  now  practised  in  European 
works  under  ordinary  conditions  never  allows  over  i  or  2  per 
cent  of  the  bacteria  of  the  raw  water  to  pass,  and  ordinarily  not 
over  one  fourth  to  one  half  of  one  per  cent,  although  exact  data 
cannot  be  obtained  owing  to  masking  effect  of  the  bacteria 
which  come  in  from  below  and  which  bear  no  relation  to  those 
of  the  raw  water.  By  increasing  the  size  of  filters,  fineness  and 


88  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

depth  of  sand  (as  at  Hamburg),  the  efficiency  can  be  materially 
increased  above  these  figures.  At  the  same  time  it  must  be 
borne  in  mind  that  the  effectiveness  of  a  filter  may  be  greatly 
impaired  by  inadequate  underdraining,  by  fluctuating  rates  of 
filtration  where  these  are  allowed,  by  freezing  in  winter  in  the 
case  of  open  filters  in  cold  climates,  and  by  other  irregularities, 
all  of  which  can  be  prevented  by  careful  attention  to  the 
respective  points. 

The  action  of  a  continuous  filter  throughout  is  mainly  that  of 
an  exceedingly  fine  strainer,  and  like  a  strainer  is  mainly  con- 
fined to  the  suspended  or  insoluble  matters  in  the  raw  water. 
The  turbidity,  sediment,  and  bacteria  of  the  raw  water  are 
largely  or  entirely  removed,  while  hardness,  organic  matter, 
and  color,  so  far  as  they  are  in  solution,  are  removed  to  only  a 
slight  extent,  if  at  all.  Hardness  can  be  removed  by  the  addi- 
tion of  lime  in  carefully  determined  quantity  before  filtration 
(Clark's  process),  by  means  of  which  the  excess  of  carbonic  acid 
in  the  water  is  absorbed  and  the  lime  added,  together  with  that 
previously  in  the  water,  is  precipitated. 

Ordinary  filtration  will  remove  from  one  fourth  to  one  third 
of  the  yellow-brown  color  of  peaty  water.  A  larger  proportion 
can  be  removed  by  the  addition  of  alum,  which  by  decomposing 
forms  an  insoluble  compound  of  alumina  with  the  coloring 
matter,  while  the  acid  of  the  alum  goes  into  the  effluent  either  as 
free  acid,  or  in  combination  with  the  lime  or  other  base  in  the 
water,  according  to  their  respective  quantities.  Freshly  pre- 
cipitated alumina  can  be  substituted  for  the  alum  at  increased 
expense  and  trouble,  and  tends  to  remove  the  color  without 
adding  acid  to  the  water.  These  will  be  discussed  more  in 
detail  in  connection  with  mechanical  filters.  Alum  is  but  rarely 
used  in  slow  sand  filtration,  the  most  important  works  where  it 
is  used  being  in  Holland  with  peaty  waters. 

After  all,  the  most  conclusive  test  of  the  efficiency  of  filtration 
is  the  healthfulness  of  the  people  who  drink  the  filtered  water ; 


THEORY  AND    EFFICIENCY   OF  CONTINUOUS  F/rfKA  TION.      89 


and  the  fact  that  many  European  cities  take  water-supplies  from 
sources  which  would  not  be  considered  fit  for  use  in  the  United 
States  and,  after  filtering  them,  deliver  them  to  populations  having 
death-rates  from  water-carried  diseases  which  are  so  low  as  to  be 
the  objects  of  our  admiration,  is  the  best  proof  of  the  efficiency 
of  carefully  conducted  filtration. 

It  is  only  necessary  to  refer  to  London,  drawing  its  water 
from  the  two  small  and  polluted  rivers,  the  Thames  and  the  Lea  ; 
to  Altona,  drawing  its  water  from  the  Elbe,  polluted  by  the 
sewage  of  6,000,000  people,  700,000  of  them  within  ten  miles  above 
the  intakes  ;  to  Berlin,  using  the  waters  of  the  Havel  and  the 
Spree  ;  to  Breslau,  taking  its  water  from  the  Oder  charged  with 
the  sewage  of  mining  districts  in  Silicia  and  Galicia,  where 
cholera  is  so  common  ;  to  Lawrence,  with  its  greatly  decreased 
death-rate  since  it  has  had  filtered  water,  and  to  the  hundred 
other  places  which  protect  themselves  from  the  infectious  mat- 
ters in  their  raw  waters  by  means  of  filtration.  A  few  of  these 
cases  are  described  more  in  detail  in  Appendices  V  to  IX,  and 
many  others  in  the  literature  mentioned  in  Appendix  X. 

An  adequate  presentation  of  even  those  data  which  have  been 
already  worked  up  and  published  would  occupy  too  much 
space.  I  think  every  one  who  has  carefully  studied  the  recent 
history  of  water  filtration  in  its  relation  to  disease  has  been 
convinced  that  filtration  carefully  executed  under  suitable  and 
normal  conditions,  even  if  not  an  absolute,  is  at  least  a  very 
substantial  protection  against  water-carried  diseases,  and  the  few 
apparent  failures  to  remove  objectionable  qualities  have  been 
without  exception  due  to  abnormal  conditions  which  are  now 
understood  and  in  future  can  be  prevented. 

BACTERIAL   EXAMINATION   OF  WATERS.  ' 

Every  large  filter-plant  should  have  arrangements  for  the 
systematic  bacterial  examination  of  the  water  before  and  after 


9O  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

filtration,  especially  where  the  raw  water  is  subject  to  serious 
pollution.  Such  examinations  need  not  be  excessively  expensive, 
and  they  will  not  only  show  the  efficiency  of  the  plant  as  a  whole, 
but  may  be  made  to  show  the  relative  efficiencies  of  the  separate 
filters,  the  relative  efficiencies  at  different  parts  of  the  periods  of 
operation,  the  effect  of  cold  weather,  etc.,  and  will  then  be  a  sub- 
stantial aid  to  the  superintendent  in  always  securing  good 
effluents  at  the  minimum  cost. 

In  addition  a  complete  record  of  the  bacteria  in  the  water  at 
different  times  may  aid  in  determining  definitely  whether  the 
water  was  connected  with  outbreaks  of  disease.  Thus  if  an  out- 
break of  disease  of  any  kind  were  preceded  at  a  certain  interval 
by  a  great  increase  in  the  number  of  bacteria, — as  has  been  the 
case,  for  example,  with  the  typhoid  epidemics  at  Altona  and 
Berlin  (see  Appendices  II  and  VII), — a  presumption  would  arise 
that  they  might  have  been  connected  with  each  other,  and  each 
time  it  was  repeated  the  presumption  would  be  strengthened, 
while,  on  the  other  hand,  outbreaks  occurring  while  the  bacteria 
remained  constantly  low  would  tend  to  discredit  such  a  theory. 

Bacterial  investigations  inaugurated  after  an  epidemic  is 
recognized,  as  has  frequently  been  done,  seldom  lead  to  results 
of  value,  both  because  the  local  normal  bacterial  conditions  are 
generally  unknown  at  the  commencement  of  the  investigation, 
and  because  the  most  important  time,  the  time  of  infection,  is 
already  long  past  before  the  first  samples  are  taken.  The  fact 
that  such  sporadic  activities  have  led  to  few  definite  results 
should  throw  no  discredit  upon  continued  observations,  which 
have  repeatedly  proved  of  inestimable  value. 

Considerable  misconception  of  the  use  of  bacterial  examina- 
tions exists.  The  simple  bacterial  count  ordinarily  used,  and  of 
which  I  am  now  speaking,  does  not  and  cannot  show  whether  a 
water  contains  disease-germs  or  not.  I  object  to  the  Chicago 
water,  not  so  much  because  a  glass  of  it  contains  a  hundred  thou- 
sand bacteria  more  or  less,  as  because  I  am  convinced,  by  a  study 


THEORY  AND    EFFICIENCY  OF  CONTINUOUS  FILTRATION.      9 1 

of  its  source  in  connection  with  the  city's  death-rate,  that  it  actu- 
ally carries  disease-germs  which  prove  injurious  to  thousands 
of  those  who  drink  it.  Now  the  fact  being  admitted  that  the 
water  is  injurious  to  health,  variations  in  the  numbers  of  bacteria 
in  the  water  drawn  from  different  intakes  and  at  different  times 
probably  correspond  roughly  with  varying  proportions  of  fresh 
sewage,  and  indicate  roughly  the  relative  dangers  from  the  use 
of  the  respective  waters.  If  filters  should  be  introduced,  the 
numbers  of  bacteria  in  the  effluents  under  various  conditions 
would  be  an  index  of  the  respective  efficiencies  of  filtration,  and 
would  serve  to  detect  poor  work,  and  would  probably  suggest 
the  measures  necessary  for  better  results. 

I  would  suggest  the  desirability  of  such  investigations  where 
mechanical  filters  are  used,  quite  as  much  as  in  connection  with 
slow  filtration ;  and  it  would  also  be  most  desirable  in  the  case  of 
many  water-supplies  which  are  not  filtered  at  all.  Such  con- 
tinued observations  have  been  made  at  Berlin  since  1884;  at 
London  since  1886;  at  Boston  and  Lawrence  since  1888;  and 
recently  at  a  large  number  of  places,  including  Chicago,  where 
observations  by  the  city  were  commenced  in  1894.  They  are  now 
required  by  the  German  Government  in  the  case  of  all  filtered 
public  water-supplies  in  Germany,  without  regard  to  the  source 
of  the  raw  water.  The  German  standard  requires  that  the  efflu- 
ent from  each  single  filter,  as  well  as  the  mixed  effluent  and  raw 
water,  shall  be  examined  daily,  making  at  some  works  10  to  30 
samples  daily.  This  amount  of  work,  however,  can  usually  be 
done  by  a  single  man  ;  and  when  a  laboratory  is  once  started,  the 
cost  of  examining  20  samples  a  day  will  not  be  much  greater 
than  if  only  20  a  week  are  taken.  In  England  and  at  some  of 
the  Continental  works  drawing  their  waters  from  but  slightly 
polluted  sources,  much  smaller  numbers  of  samples  are  examined. 

The  question  whether  the  examinations  should  be  made  under 
the  direction  of  the  water-works  company  or  department,  or  by 
an  independent  body — as,  for  instance,  by  the  Board  of  Health — 


92  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

will  depend  upon  local  conditions.  The  former  arrangement 
gives  the  superintendent  of  the  filters  the  best  chance  to  study 
their  action,  as  he  can  himself  control  the  collection  of  samples 
in  connection  with  the  operation  of  the  filters,  and  arrange  them 
to  throw  light  upon  the  points  he  wishes  to  investigate ;  while 
examination  by  a  separate  authority  affords  perhaps  greater 
protection  against  the  possible  carelessness  or  dishonesty  of 
water-works  officials.  An  arrangement  being  adopted  in  many 
cases  in  Germany  is  to  have  a  bacterial  laboratory  at  the  works 
which  is  under  the  control  of  the  superintendent,  and  in  which 
the  very  numerous  compulsory  observations  are  made,  while  the 
Board  of  Health  causes  to  be  examined  from  time  to  time  by 
its  own  representatives,  who  have  no  connection  with  the  water- 
works, samples  taken  to  check  the  water-works  figures,  as  well 
as  to  show  the  character  of  the  water  delivered. 

It  seems  quite  desirable  to  have  a  man  whose  principal  busi- 
ness is  to  make  these  examinations ;  as  in  case  he  also  has  numer- 
ous other  duties,  the  examinations  may  be  found  to  have  been 
neglected  at  some  time  when  they  were  most  wanted.  Such  a 
man  should  have  had  thorough  training  in  the  principles  of  bac- 
terial manipulation,  but  it  is  quite  unnecessary  that  he  should  be 
an  expert  bacteriologist,  especially  if  a  competent  bacteriologist 
is  retained  for  consultation  in  cases  of  doubt  or  difficulty. 


INTERMITTENT  FILTRATION.  93 


CHAPTER  VII. 
INTERMITTENT   FILTRATION. 

BY  intermittent  filtration  is  understood  that  filtration  in 
which  the  filtering  material  is  systematically  and  adequately 
ventilated,  and  where  the  water  during  the  course  of  filtration  is 
brought  in  contact  with  air  in  the  pores  of  the  sand.  In  con- 
tinuous filtration,  which  alone  has  been  previously  considered, 
the  air  is  driven  out  of  the  sand  as  completely  as  possible  before 
the  commencement  of  filtration,  and  the  sand  is  kept  continu- 
ously covered  with  water  until  the  sand  becomes  clogged  and  a 
draining,  with  an  incidental  aeration,  is  necessary  to  allow  the 
filter  to  be  scraped  and  again  put  in  service. 

In  intermittent  filtration,  on  the  other  hand,  water  is  taken 
over  the  top  of  the  drained  sand  and  settles  into  it,  coming  in 
contact  with  the  air  in  the  pores  of  the  sand,  and  passes  freely 
through  to  the  bottom  when  the  water-level  is  kept  well  down. 
After  a  limited  time  the  application  of  water  is  stopped,  and  the 
filter  is  allowed  to  again  drain  and  become  thoroughly  aerated 
preparatory  to  receiving  another  dose  of  water. 

This  system  of  treating  water  was  suggested  by  the  un- 
equalled purification  of  sewage  effected  by  a  similar  treatment. 
It  has  been  investigated  at  the  Lawrence  Experiment  Station, 
and  applied  to  the  construction  of  a  filter  for  the  city  of  Law- 
rence, both  of  which  are  due  to  the  indefatigable  energy  of 
Hiram  F.  Mills,  C.E. 

In  its  operation  intermittent  differs  from  continuous  filtration 
in  that  the  straining  action  is  less  perfect,  because  the  filters 
yield  no  water  while  being  aerated,  and  must  therefore  filter  at  a 
greater  velocity  when  in  use  to  yield  the  same  quantity  of  water 
in  a  given  time,  and  also  on  account  of  the  mechanical  disturb- 


94  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

ance  which  is  almost  invariably  caused  by  the  application  of  the 
water ;  but,  on  the  other  hand,  the  oxidizing  powers  of  the  filter, 
or  the  tendency  to  nitrify  and  destroy  the  organic  matters,  are 
stronger,  and  in  addition,  if  the  rate  is  not  too  high,  the  bacteria 
die  more  rapidly  in  the  thoroughly  aerated  sand  than  is  the  case 
with  ordinary  filters. 

It  was  found  at  Lawrence  in  connection  with  sewage  filters 
that  when  nitrification  was  actively  taking  place  the  numbers  of 
bacteria  were  much  lower  than  under  opposite  conditions,  and  it 
was  thought  that  nitrification  in  itself  might  cause  the  death  of 
the  bacteria.  Later  experiments,  however,  with  pure  cultures 
of  bacteria  of  various  kinds  applied  to  intermittent  filters 
with  water  to  which  ammonia  and  salts  suitable  for  nitrification 
were  added,  showed  that  bacteria  of  all  the  species  tried  were 
able  to  pass  the  filter  in  the  presence  of  nitrification,  producing 
at  least  one  thousand  times  as  much  nitrates  as  could  result  in 
any  case  of  water-filtration,  as  freely  as  was  the  case  when  the 
ammonia  was  not  added  and  there  was  but  little  nitrification. 
These  results  showed  conclusively  that  nitrification  in  itself  is 
not  an  important  factor  in  bacterial  removal,  although  nitrifica- 
tion and  bacterial  purification  do  to  some  extent  go  together  ; 
perhaps  in  part  because  the  nitrification  destroys  the  food  of 
the  bacteria  and  so  starves  them  out,  but  probably  much  more 
because  the  conditions  of  aeration,  temperature,  etc.,  which 
favor  nitrification  also  favor  equally,  and  even  in  its  absence, 
the  death  of  the  bacteria. 

The  rate  at  which  water  must  pass  through  an  intermittent 
filter  is,  on  account  of  the  intervals  of  rest,  considerably  greater 
than  that  required  to  give  a  corresponding  total  yield  from  a 
continuous  filter,  and  its  straining  effect  is  reduced  to  an  extent 
comparable  to  this  increase  in  rate  ;  and  if  other  conditions  did 
not  come  in,  the  bacterial  efficiency  of  an  intermittent  filter 
would  remain  below  that  of  a  continuous  one. 

As  a  matter  of  fact  the  bacterial  efficiency  has  usually  been 


INTERMITTENT  FILTRATION.  95 

found  to  be  less  with  intermittent  filters  at  the  Lawrence  Ex- 
periment Station,  when  they  have  been  run  at  rates  such  as  are 
commonly  used  for  continuous  filters  in  Europe,  say  from  one 
and  one  half  to  two  million  gallons  and  upwards  per  acre  daily. 
With  lower  rates,  and  especially  with  rather  fine  materials,  the 
bacterial  efficiency  is  much  greater;  but  it  may  be  doubted 
whether  it  would  ever  be  greater  than  that  of  a  continuous  filter 
with  the  same  filtering  material  and  the  same  total  yield  per 
acre.  The  number  of  bacteria  coming  from  the  underdrains 
is  apparently  generally  less,  and  with  very  high  summer  tem- 
peratures much  less,  than  in  continuous  filters,  and  this  often 
gives  an  apparent  bacterial  superiority  to  the  intermittent  filters. 

The  effluents  from  intermittent  often  contain  less  slightly  or- 
ganic matter  than  those  from  continuous  filters  ;  but,  on  the  other 
hand,  hardly  any  water  proposed  for  a  public  water-supply  has 
organic  matter  enough  to  be  of  any  sanitary  significance  what- 
ever, apart  from  the  living  bodies  which  often  accompany  it ; 
and  if  the  latter  are  removed  by  straining  or  otherwise,  we  can 
safely  disregard  the  organic  matters.  In  addition,  the  water 
filtered  will  in  a  great  majority  of  cases  have  enough  air  dis- 
solved in  itself  to  produce  whatever  oxidation  there  is  time  for 
in  the  few  hours  required  for  it  to  pass  the  filter,  and  it  is  only 
at  very  low  rates  of  filtration  that  intermittent  filters  produce 
effluents  of  greater  chemical  purity  than  by  the  ordinary  process. 
The  yellow-brown  coloring  matter  present  in  so  many  waters 
appears  to  be  quite  incapable  of  rapid  nitrification ;  and  where  it 
is  to  some  extent  removed  by  filtration,  the  action  is  dependent 
upon  other  and  but  imperfectly  understood  causes  which  seem 
to  act  equally  in  continuous  and  intermittent  filters. 

The  peculiarities  of  construction  involved  by  this  method  of 
filtration  will  be  best  illustrated  by  a  discussion  of  the  Lawrence 
city  filter  designed  by  Hiram  F.  Mills,  C.E.,  which  is  the  only 
filter  in  existence  upon  this  plan.* 

*  I  am  informed  that  several  other  filters  upon  the  same  principle  have  been  more 
recently  built. 


96  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


THE   LAWRENCE  FILTER. 

The  filter  consists  of  a  single  bed  2\  acres  in  area,  the  bot- 
tom of  which  is  7  feet  below  low  water  in  the  river,  and  filled 
with  gravel  and  sand  to  an  average  depth  of  4^  feet.  The  filter 
is  all  in  a  single  bed  instead  of  being  divided  into  the  three  or 
four  sections  which  would  probably  have  been  used  for  a  contin- 
uous filter  of  this  size.  The  water-tight  bottom  also  was  dis- 
pensed with,  and  the  gravel  was  prevented  from  sinking  into  the 
silt  by  thin  intermediate  layers  of  graded  materials.  The  saving 
in  cost  was  considerable ;  but,  on  the  other  hand,  a  considerable 
quantity  of  ground-water  comes  up  through-  the  bottom  and  in- 
creases the  hardness  of  the  water  from  1.5  to  2.6  parts  of  calcium 
carbonate  in  100,000;  and  while  the  water  when  compared  with 
many  other  waters  is  still  extremely  soft,  the  addition  cannot  be 
regarded  as  desirable.  The  ground-water  also  contains  iron, 
which  increases  the  color  of  the  water  above  what  it  would 
otherwise  be. 

The  underdrains  have  a  frictional  resistance  ten  times  as 
great  as  would  be  desirable  for  a  continuous  filter,  the  idea 
being  to  check  extreme  rates  of  filtration  in  case  of  unequal 
flooding,  and  also  to  limit  the  quantity  of  water  which  could  be 
gotten  through  the  filter  to  that  corresponding  to  a  moderate 
rate  of  filtration. 

The  sand,  instead  of  being  all  of  the  same-sized  grain,  is  of 
two  grades,  with  effective  sizes  respectively  0.25  and  0.30  mm., 
the  coarser  sand  being  placed  farthest  away  from  the  under- 
drains, where  its  greater  distance  is  intended  to  balance  its 
reduced  frictional  resistance  and  make  all  parts  filter  at  an 
equal  rate. 

The  surface  instead  of  being  level  is  waved,  that  is,  there  are 
ridges  thirty  feet  apart,  sloping  evenly  to  the  valleys  one  foot 
deep  half  way  between  them,  to  allow  water  to  be  brought  on 


INTERMITTENT  FILTRATION.  97 

rapidly  without  disturbing-  the  sand  surface.  For  the  same 
reason,  as  well  as  to  secure  equality  of  distribution,  a  system  of 
concrete  carriers  for  the  raw  water  goes  to  all  parts  of  the  filter, 
reducing  the  effective  filtering  area  by  4  or  5  per  cent.  The 
filter  is  scraped  as  necessary  in  sections,  the  work  being  per- 
formed when  the  filter  is  having  its  daily  rest  and  aeration. 
Owing  to  the  difference  in  frictional  resistance  before  and  after 
scraping,  and  to  the  fact  that  it  is  impossible  to  scrape  the  entire 
area  in  one  day,  considerable  variations  in  the  rate  of  filtration 
in  different  parts  of  the  filter  must  occur.  The  heavy  frictional 
resistance  of  the  underdrains  when  more  than  the  proper 
quantity  of  water  passes  them  tends  to  correct  this  tendency 
especially  for  the  more  remote  parts  of  the  filter,  but  perhaps 
at  the  expense  of  those  near  to  the  main  drain. 

The  filter  is  not  covered  as  the  suggestions  in  Chapter  II 
would  require,  but  this  is  hardly  on  account  of  its  being  an  in- 
termittent filter. 

The  annual  report  of  the  Massachusetts  State  Board  of 
Health  for  1893  states  that  during  the  first  half  of  December, 
1893,  the  surface  remained  covered,  that  is,  it  was  used  continu- 
ously, and  after  December  i6th  it  was  so  used  when  the  temper- 
ature was  below  24°,  and  was  drained  only  when  the  tempera- 
ture was  24°  or  above.  The  days  on  which  the  filter  was  drained 
during  the  remainder  of  December  are  not  given,  but  during 
January  and  February,  1894,  the  filter  remained  covered  29  days 
and  was  drained  30  days.  Bacterial  samples  were  taken  on  44 
of  these  days,  22  days  when  it  was  drained  and  22  when  it  was 
not.  The  average  number  of  bacteria  on  the  days  when  it  was 
not  drained  was  137  and  on  those  days  when  it  was  drained  252 
per  cubic  centimeter. 

From  February  24th  to  March  I2th  the  number  of  bacteria 
were  unusually  high,  averaging  492  per  cubic  centimeter,  or  5.28 
per  cent  of  the  9308  applied.  During  this  period  the  filter  was 
used  intermittently ;  there  was  ice  upon  it,  and  parts  of  the  sur- 


98  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

face  were  scraped  under  the  ice,  and  high  rates  of  filtration 
undoubtedly  resulted  on  the  scraped  areas.  After  March  I2th 
the  ice  had  disappeared  and  very  much  better  results  were  ob- 
tained. 

While  there  may  be  some  question  as  to  the  direct  cause  of 
this  decreased  efficiency  with  continued  cold  weather  and  ice, 
the  results  certainly  are  not  such  as  to  show  the  advisability  of 
building  open  filters  in  the  Lawrence  climate. 

The  cost  of  building  the  filter  in  comparison  with  European 
filters  was  extraordinarily  low — only  $67,000,  or  $27,000  per  acre 
of  filter  surface.  To  have  constructed  open  continuous  filters 
of  the  same  area  with  water-tight  bottoms,  divided  into  sections 
with  separate  drains  and  regulating  apparatus,  with  the  necessary 
piping,  would  have  cost  at  least  half  as  much  more,  and  with  the 
masonry  cover  which  I  regard  as  most  desirable  in  the  Lawrence 
climate  the  cost  would  have  been  two  or  three  times  the  ex- 
penditure actually  required. 

It  was  no  easy  matter  to  secure  the  consent  of  the  city  gov- 
ernment to  the  expenditure  of  even  the  sum  used  ;  there  was 
much  skepticism  as  to  the  process  of  filtration  in  general,  and  it 
was  said  that  mechanical  filters  could  be  put  in  for  about  the 
same  cost.  Insisting  upon  the  more  complete  and  expensive 
form  might  have  resulted  either  in  an  indefinite  postponement  of 
action,  or  in  the  adoption  of  an  inferior  and  entirely  inadequate 
process.  Still  I  feel  strongly  that  in  the  end  the  greater  ex- 
pense would  have  proved  an  excellent  investment  in  securing 
softer  water  and  in  the  greater  facility  and  security  of  operating 
the  filter  in  winter. 

In  regard  to  the  effect  of  the  Lawrence  filter  upon  the  health 
of  the  city,  I  can  best  quote  from  Mr.  Mills'  paper  in  the  Report 
of  the  Massachusetts  State  Board  of  Health  for  1893,  and  also 
published  in  the  Journal  of  the  New  England  Water-works 
Association.  Mr.  Mills  says :  "  In  the  following  diagram  [Fig. 
15]  the  average  number  of  deaths  from  typhoid  fever  at  Law- 


INTERMITTENT  FILTRATION. 


99 


rence  for  each  month  from  October  to  May,  in  the  preced- 
ing five  years,  are  given  by  the  heavy  dotted  line ;  and  the 
number  during  the  past  eight  months  are  given  by  the  heavy 
full  line. 

"  The  total  number  for  eight  months  in  past  years  has  been 
forty-three,  and  in  the  present  year  seventeen,  making  a  saving 
of  twenty-six.  Of  the  seventeen  who  died  nine  were  operatives 


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ct.       Nor.         Dec.        Jan.         Feb.        Mar.         Apr.       Ma 
FIG.  15.  —  TYPHOID  FEVER  IN  LAWRENCE. 

in  the  mills,  each  of  whom  was  known  to  have  drunk  unfiltered 
canal  water,  which  is  used  in  the  factories  at  the  sinks  for 
.washing. 

"  The  finer  full  line  shows  the  number  of  those  who  died 
month  after  month  who  are  not  known  to  have  used  the 
poisoned  canal  water.  The  whole  number  in  the  eight  months 
is  eight. 

"  It  is  evident  from  the  previous  diagram  [not  reproduced] 
that  the  numbers  above  the  fine  full  line,  here,  follow  after  those 
at  Lowell  in  the  usual  time,  and  were  undoubtedly  caused  by  the 
sickness  at  Lowell ;  but  we  have  satisfactory  reason  to  conclude 
that  the  disease  was  not  propagated  through  the  filter  but  that 
the  germs  were  conveyed  directly  into  the  canals  and  to  those 
who  drank  of  the  unfiltered  canal  water.  Among  the  operatives 


100  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

of  one  of  the  large  corporations  not  using  the  canal  water  there 
was  not  a  case  of  typhoid  fever  during  this  period.  Warnings 
have  been  placed  in  the  mills  where  canal  water  is  used  to 
prevent  the  operatives  from  drinking  it. 

"  We  find,  then,  that  the  mortality  from  typhoid  fever  has, 
during  the  use  of  the  filter,  been  reduced  to  40  per  cent  of  the 
former  mortality,  and  that  the  cases  forming  nearly  one  half  of 
this  40  per  cent  were  undoubtedly  due  to  the  continued  use  of 
unfiltered  river  water  drawn  from  the  canals." 

The  results  for  the  remainder  of  1894  have  been  equally 
favorable,  and  show  conclusively  the  value  of  filtration. 


CHEMNITZ  WATER-WORKS. 

The  only  other  place  which  I  have  found  where  anything 
approaching  intermittent  filtration  of  water  is  systematically 
employed  is  Chemnitz,  Germany.  The  method  there  used  bears 
the  same  relation  to  intermittent  filtration  as  does  broad  irriga- 
tion of  sewage  to  the  corresponding  method  of  sewage  treatment; 
that  is,  the  principles  involved  are  mainly  the  same,  but  a  much 
larger  filtering  area  is  used,  and  the  processes  take  place  at  a 
lower  rate  and  under  less  close  control. 

The  water- works  were  built  about  twenty  years  ago  by  plac- 
ing thirty-nine  wells  along  the  Zwonitz  River,  connected  by 
siphon  pipes,  with  a  pumping-station  which  forced  the  water  to 
an  elevated  reservoir  near  the  city  (Fig.  16).  The  wells  are  built 
of  masonry,  5  or  6  feet  in  diameter  and  10  or  12  feet  deep,  and 
are  on  the  rather  low  bank  of  the  river.  The  material,  with  the 
exception  of  the  surface  soil,  and  loam  about  3  feet  deep,  is  a 
somewhat  mixed  gravel  with  an  effective  size  of  probably  from 
0.2^5  to  0.50  mm.,  so  that  water  is  able  to  pass  through  it  freely. 
The  wells  are,  on  an  average,  about  120  feet  apart,  and  the  line 
is  seven  eighths  of  a  mile  long. 


INTERMITTENT  FILTRATION.  1OI 

It  was  found  that  in  dry  times  the  groundrwater  level  in  the 
entire  neighborhood  was  lowered  some  feet  below  the  level  of 
the  river  without  either  furnishing  water  enough  or  stopping 
the  flow  of  the  river  below.  The  channel  of  the  river  was  so 
silted  that,  notwithstanding  the  porous  material,  the  water  could 
not  penetrate  it  to  go  toward  the  wells. 


I  Feet 

FIG.  16.— PLAN  OF  AREA  USED  FOR  INTERMITTENT  FILTRATION  AT  CHEMNITZ. 

A  dam  was  now  built  across  the  river  near  the  pumping- 
station,  and  a  canal  was  dug  from  above  the  dam,  crossing  the 
line  of  wells  and  running  parallel  to  it  on  the  back  side  for  about 
half  a  mile.  Later  a  similar  canal  was  dug  back  of  the  remain- 
ing upper  wells.  Owing  to  the  difference  in  level  in  the  river 
above  and  below,  the  canals  can  be  emptied  and  filled  at 
pleasure.  They  are  built  with  carefully  prepared  sand  bottoms, 
and  the  sand  sides  are  protected  by  an  open  paving,  to  allow  the 
percolation  of  as  much  water  as  possible,  and  the  sand  is  cleaned 
by  scraping,  as  is  usual  with  ordinary  sand  filters,  once  a  year  or 
oftener. 

The  yield  from  the  wells  was  much  increased  by  these  canals, 
but  the  water  of  the  river  is  polluted  to  an  extent  which  would 
ordinarily  quite  prevent  even  the  thought  of  its  being  used  for 
water-supply,  and  it  was  found  that  the  water  going  into  the 
ground  from  the  canals,  and  passing  through  the  always  satu- 
rated gravel  to  the  wells,  without  coming  in  contact  with  air  at 


102  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

any  point,  after  a  time  contained  iron  and  had  an  objectionable 
odor. 

To  avoid  this  disagreeable  result  the  meadow  below  the 
pumping-station  was  laid  out  as  an  irrigation  field  (Fig.  16). 
The  water  from  above  the  dam  was  taken  by  a  canal  on  the 
opposite  side  of  the  river  through  a  sedimentation  pond  (which, 
however,  is  not  now  believed  to  be  necessary  and  is  not  always 
used),  and  then  under  the  river  by  a  siphon  to  a  slightly  ele- 
vated point  on  the  meadow,  from  which  it  is  distributed  by 
a  system  of  open  ditches,  exactly  as  in  sewage  irrigation.  The 
area  irrigated  is  not  exactly  defined  and  varies  somewhat  from 
time  to  time  ;  the  rate  of  filtration  may  be  roughly  estimated 
at  from  100,000  to  150,000  gallons  per  acre  daily,  although 
limited  portions  may  occasionally  get  five  times  these  quantities 
for  a  single  day.  The  water  passes  through  the  three  feet  of 
soil  and  loam,  and  afterward  through  an  average  of  six  feet 
of  drained  coarse  sand  or  gravel  in  which  it  meets  air,  and 
afterward  filters  laterally  through  the  saturated  gravel  to  the 
wells.  The  water  so  obtained  is  invariably  of  good  quality  in 
every  way,  colorless,  free  from  odor  and  from  bacteria.  The 
surface  of  the  irrigated  land  is  covered  with  grass  and  has  fruit- 
trees  (mostly  apple)  at  intervals  over  its  entire  area. 

This  first  system  of  irrigation  is  entirely  by  gravity.  On 
account  of  natural  limits  to  the  land  it  could  not  be  conveniently 
extended  at  this  point,  and  to  secure  more  area,  the  higher  land 
above  the  pumping-station  was  being  made  into  an  irrigation 
field  in  1894.  This  is  too  high  to  be  flooded  by  gravity,  and  will 
be  used  only  for  short  periods  in  extremely  dry  weather.  The 
water  is  elevated  the  few  feet  necessary  by  a  gas-engine  on  the 
river-bank.  In  times  of  wet  weather  enough  water  is  obtained 
from  the  wells  without  irrigation,  and  the  land  is  only  irrigated 
when  the  ground-water  level  is  too  low. 

During  December,  January,  and  February  irrigation  is 
usually  impossible  on  account  of  temperature,  and  the  canals  are 


INTERMITTENT  FILTRATION.  1 03 

then  used,  keeping  them  filled  with  water  so  that  freezing  to  the 
bottom  is  impossible ;  but  trouble  with  bad  odors  in  the  filtered 
water  drawn  from  the  wells  is  experienced  at  these  times. 

The  drainage  area  of  the  Zwonitz  River  is  only  about  44 
square  miles,  and  upon  it  are  a  large  number  of  villages  and 
factories,  so  that  the  water  is  excessively  polluted.  The  water 
in  the  wells,  however,  whether  coming  from  natural  sources,  or 
from  irrigation,  or  from  the  canals,  has  never  had  as  many  as  100 
bacteria  per  cubic  centimeter,  and  is  regarded  as  entirely 
wholesome. 

In  extremely  dry  weather  the  river,  even  when  it  is  all  used 
for  irrigation  so  that  hardly  any  flows  away  below,  cannot  be 
made  to  supply  the  necessary  daily  quantity  of  2,650,000  gal- 
lons, and  to  supply  the  deficiency  at  such  times,  as  well  as  to 
avoid  the  use  of  the  canals  in  winter,  a  storage  reservoir  hold- 
ing 95,000,000  gallons  has  recently  been  built  on  a  feeder  of  the 
river.  This  water,which  is  from  an  uninhabited  drainage  area, 
is  filtered  through  ordinary  continuous  filters  and  flows  to  the 
city  by  gravity.  Owing  to  the  small  area  of  the  watershed  it  is 
incapable  of  supplying  more  than  a  fraction  of  the  water  for  the 
city,  arid  will  be  used  to  supplement  the  older  works. 

This  Chemnitz  plant  is  of  especial  interest  as  showing  the 
successful  utilization  of  a  river-water  so  grossly  polluted  as  to 
be  incapable  of  treatment  by  the  ordinary  methods.  Results 
obtained  at  the  Lawrence  Experiment  Station  have  shown  that 
sewage  is  incapable  of  being  purified  by  continuous  filtration, 
the  action  of  air  being  essential  for  a  satisfactory  result.  With 
ordinary  waters  only  moderately  polluted"  this  is  not  so  ;  for 
they  carry  enough  dissolved  air  to  effect  their  own  purifica- 
tion. In  Chemnitz,  however,  as  shown  by  the  results  with  the 
canals,  the  pollution  is  so  great  that  continuous  filtration  is 
inadequate  to 'purify  the  water,  and  the  intermittent  filtration 
adopted  is  the  only  method  likely  to  yield  satisfactory  results  in 
such  cases. 


104  FILTRATION  OF  PUBLIC  WATER-SUPPLIED 

Intermittent  filtration  is  now  being  adopted  for  purifying 
brooks  draining  certain  villages  and  discharging  into  the  ponds  '< 
or  reservoirs  from  which  Boston  draws  its  water-supply.  Thej 
water  of  Pegan  Brook  below  Natick  has  been  so  filtered  since 
1893  with  most  satisfactory  results,  and  affords  almost  absolute 
protection  to  Boston  from  any  infection  which  might  otherwise 
enter  the  water  from  that  town.  A  similar  treatment  is  soon  to 
be  given  to  a  brook  draining  the  city  of  Marlborough.  The 
sewage  from  these  places  is  not  discharged  into  the  brooks,  but 
is  otherwise  provided  for,  but  nevertheless  they  receive  many 
polluting  matters  from  the  houses  and  streets  upon  their  banks. 

The  filtration  used  resembles  in  a  measure  that  at  Chemnitz, 
and  I  am  informed  by  the  engineer,  Mr.  Desmond  FitzGerald, 
that  it  was  adopted  on  account  of  its  convenience  for  this  partic- 
ular problem,  and  not  because  he  attaches  any  special  virtue  to 
the  intermittent  feature. 

APPLICATION  OF  INTERMITTENT  FILTRATION. 

In  regard  to  the  use  of  waters  as  grossly  polluted  as  the 
Zwonitz,  the  tendency  is  strongly  to  avoid  their  use,  no  matter 
how  complete  the  process  of  purification  may  be;  but  in  case  it 
should  be  deemed  necessary  to  use  so  impure  a  water  fora  public 
supply,  intermittent  filtration  is  the  only  process  known  which 
would  adequately  purify  it.  And  it  should  be  used  at  compara- 
tively low  rates  of  filtration.  I  believe  that  an  attempt  to  filter 
the  Zwonitz  at  the  rate  used  for  the  Merrimac  water  at  Law- 
rence, which  is  by  comparison  but  slightly  polluted,  would  re- 
sult disastrously. 

The  operation  in  winter  must  also  be  considered.  Intermit- 
tent filtration  of  sewage  on  open  fields  in  Massachusetts  winters 
is  only  possible  because  of  the  comparatively  high  temperature 
of  the  sewage  (usually  40°  to  50°),  and  would  be  a  dismal  failure 
with  sewage  at  the  freezing-point,  the  temperature  to  be  ex- 
pected in  river-waters  in  winter. 


INTERMITTENT  FILTRATION.  IO$ 

It  is  impossible  to  draw  a  sharp  line  between  those  waters 
which  are  so  badly  polluted  as  to  require  intermittent  filtration 
for  their  treatment  and  those  which  are  susceptible  to  the  ordi- 
nary continuous  filtration.  Examples  of  river-waters  polluted 
probably  beyond  the  limits  reached  in  any  American  waters 
used  for  drinking  purposes  and  successfully  filtered  with  contin- 
uous filters  are  furnished  by  Altona,  Breslau,  and  London. 

Intermittent  filtration  may  be  considered  in  those  cases 
where  it  is  proposed  to  use  a  water  polluted  entirely  beyond  the 
ordinary  limits,  and  for  waters  containing  large  quantities  of  de- 
composable organic  matters  and  microscopical  organisms ;  but  in 
those  cases  where  a  certain  and  expeditious  removal  of  mud  is 
desired,  and  where  waters  are  only  moderately  polluted  by 
sewage,  but  still  in  their  raw  state  are  unhealthy,  it  is  not  ap- 
parent that  intermittent  filtration  has  any  advantages  commen- 
surate with  the  disadvantages  of  increased  rate  to  produce  the 
same  total  yield  and  of  the  increased  difficulty  of  operation, 
particularly  in  winter ;  and  in  such  cases  continuous  filtration  is 
to  be  preferred. 


IO6  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


CHAPTER  VIII. 

OTHER  METHODS  OF  FILTRATION. 

MECHANICAL  FILTERS   WITHOUT   COAGULENTS. 

THE  mechanical  filters  so  largely  used  in  the  United  States 
to  clarify  water  for  manufacturing  purposes  consist,  in  general, 
of  iron  cylinders  filled  with  sand  through  which  water  is  forced  at 
rates  of  from  one  to  three  hundred  million  gallons  per  acre  daily, 
or  we  may  say,  in  a  general  way, 'one  hundred  times  as  fast  as  in 
the  European  filters  for  public  water-supplies.  These  filters,  of 
which  there  are  many  patented  kinds  upon  the  market*  differ 
from  each  other  mainly  in  the  method  with  which  the  clogging 
of  the  sand  is  removed.  Instead  of  scraping,  as  in  the  case  of  the 
slow  sand-filters  at  intervals  of  some  weeks,  the  whole  body  of 
the  sand  is  washed  in  the  filter  itself  at  short  intervals,  depend- 
ing upon  the  rapidity  of  clogging. 

I  wish  to  emphasize  specially  the  fact  that  the  subject  of  the 
various  patents  and  the  differences  between  the  different  makes 
of  filters  and  between  the  whole  class  of  mechanical  filters  and 
slow  sand-filters  lie  in  the  methods  of  cleaning  the  sand  and 
regulating  the  rate,  pressure,  etc.  So  far  as  purification  is  con- 
cerned, the  principles  of  these  various  filters  are  identical  with 
each  other  and  with  those  governing  the  action  of  slow  filters. 
The  failure  to  grasp  this  fundamental  point  has  led  to  not  a  little 
misunderstanding  in  regard  to  them. 

Mechanical  filters  were  originated  in  paper-mills  to  remove 
from  the  vast  volumes  of  water  required  those  comparatively 
large  particles  which  would  otherwise  affect  the  appearance  or 
texture  of  the  paper.  Exactly  how  large  a  particle  must  be  to 


OTHER   METHODS   OF  FILTRATION.  IO/ 

injure  the  appearance  of  paper  I  do  not  know  ;  but  I  can  hardly 
think  that  anything  less  than  one  thousandth  of  an  inch  in 
diameter  would  be  objectionable,  while  the  particles  which  cause 
turbidity  in  drinking-water  as  well  as  the  germs  of  disease  are 
often  less  than  one  tenth  of  this  length.  The  cost  of  filtration 
which  removes  particles  one  ten-thousandth  of  an  inch  long  is 
unquestionably  greater  than  that  which  only  removes  particles 
one  thousandth  of  an  inch  long  ;  and  if  the  latter  suffices  for 
paper-making,  a  manufacturer  will  have  reason  to  be  satisfied 
with  it,  and  will  not  insist  upon  the  more  thorough  and  expensive 
treatment.  The  argument  does  not  apply  to  a  city  water-supply, 
where  the  requirements  are  radically  different. 

From  what  has  been  said  in  Chapter  IV  it  would  follow  that 
filtration  at  any  such  rates  as  are  invariably  followed  in  mechani- 
cal filters,  and  are  necessary  with  them,  on  account  of  the  high 
relative  cost  of  the  effective  filtering  area,  would  be  quite  incap- 
able of  removing  the  bacteria.  And  this  is  the  case  so  long  as 
the  use  of  alum  or  other  coagulents  is  not  considered.  I  do  not 
know  of  any  evidence  whatever  that  mechanical  filters  without 
the  use  of  alum  effect  the  removal  of  more  than  an  unimportant 
fraction  of  the  bacteria.  My  own  experience  with  filters  of  this 
class  has  never  shown  the  removal  of  more  than  from  ten  to  fifty 
per  cent  of  bacteria,  and  I  regard  this  as  a  fair  estimate  of  their 
bacterial  efficiency  in  general,  although  it  is  undoubtedly  true 
that  samples  taken  just  before  the  cleaning  of  the  filter  may 
show  much  better  results. 

Moreover,  1  have  received  direct  evidence  that  polluted 
water  filtered  in  this  way  is  not  rendered  free  from  infection,  but 
is  capable  of  causing  disease  apparently  as  freely  as  the  same 
water  would  do  without  filtration. 

A  village  in  Northern  Vermont,  has  two  water-supplies. 
About  half  the  population  is  supplied  by  an  aqueduct  com- 
pany from  a  pond  which  is  comparatively  free  from  pollu- 
tion, and  in  addition  the  water  is  filtered  through  three  filters  of 


108  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

fine  sand,  at  a  rate  of  six  million  gallons  per  acre  daily,  or  less. 
The  filters  are  covered  as  a  protection  against  frost.  The  other 
or  public  supply  is  drawn  from  a  small  river  flowing  through  the 
villiage  and  from  a  point  within  the  village  limits.  A  town  with 
2000  inhabitants,  is  nine  miles  above,  and  another  village  with  a 
considerable  although  smaller  population,  is  only  three  miles 
away  ;  and  within  the  limits  of  the  village  itself  there  are  a  num- 
ber of  houses  standing  directly  upon  the  banks  of  the  river,  and 
which  undoubtedly  drain  into  it,  although  there  are  no  public 
sewers  discharging  into  the  river  above  the  water  intake.  The 
obvious  sources  of  pollution  of  the  town  supply  caused  anxiety 
in  regard  to  its  quality,  and  to  improve  the  supply  a  mechanical 
filter  of  one  of  the  leading  makes  was  purchased,  and  was  put  in 
operation  Sept.  10,  1892.  No  alum  was  used. 

During  the  following  winter  and  spring  there  was  a  severe 
epidemic  of  typhoid  fever.  With  a  population  of  about  6000 
there  were  150  cases  and  30  deaths.  The  Health  Officer  foun 
upon  examination  that  of  these,  135  cases  were  among  those  wh 
were  regularly  supplied  with  the  town  water,  and  that  of  the 
15  cases  among  the  half  of  the  population  supplied  by  the  aque- 
duct company  a  considerable  number  had  access,  at  their  places 
of  business  or  elsewhere,  to  the  town  supply. 

The  epidemic  was  caused  beyond  a  reasonable  doubt  by  the 
use  of  the  polluted  river  water-supply,  and  the  fact  that  it  wa 
filtered  by  a  mechanical  filter,  without  alum,  at  a  high  rate,  w 
no  protection  to  the  town. 

It  is,  after  all,  not  a  great  exaggeration  to  say  that  in  filterin 
public  water-supplies  the  use  of  a  mechanical  filter  without  c 
agulent  only  "  takes  the  logs  out,"  for  it  certainly  fails  to  remov 
those  substances  which  are  most  objectionable  in  drinking-wate 
and   which  are  capable  of   being  removed  by  better  forms  o 
filtration. 

Two  reasons  are  apparent  which  account  to  some  extent  fo: 
the  use  of  this  form  of  filtration  for  public  water-supplies.     Fi 


OTHER   METHODS   OF  FILTRATION.  1 09 

the  removal  of  the  visible  floating  or  suspended  particles  which 
are  the  most  obvious  foreign  matters  in  the  water,  and  the  re- 
moval of  which  is  too  often  taken  by  ignorant  persons  for  an 
index  of  the  removal  of  other  matters  which  cannot  be  so  readily 
observed ;  and,  second,  the  sight  of  large  quantities  of  dirt  found 
in  the  filter  after  a  run,  and  which  were  obviously  removed  from 
the  water,  implying  that  the  latter  became  purified  by  losing  so 
much  filth.  A  little  thought  will  show  that  this  is  exactly  like 
arguing  that  because  a  pail  of  water  was  taken  out  of  the  pond 
therefore  the  pond  is  now  dry. 

The  whole  subject  of  mechanical  filtration  without  coagulents 
may  be  summed  up  by  saying  that  any  water  not  suitable  for 
drinking  raw  will  still  be  almost  equally  objectionable  when  fil- 
tered in  this  way. 

THE   USE   OF  ALUM. 

The  addition  of  a  small  quantity  of  alum  or  sulphate  of 
alumina  to  water  before  filtration  is  often  used  in  connection  with 
mechanical  filtration,  and  introduces  an  entirely  new  factor. 

The  alum  is  decomposed  into  its  component  parts,  sulphuric 
acid  and  alumina,  the  former  of  which  combines  with  the  lime  or 
other  base  present  in  the  water,  or,  in  case  enough  of  this  is  lack- 
ing, it  remains  partly  as  free  acid  and  partly  as  undecomposed 
alum,  while  the  alumina  forms  a  gelatinous  precipitate  which 
draws  together  and  surrounds  the  suspended  matters  present  in 
the  water,  including  the  bacteria,  and  allows  them  to  be  much 
more  easily  removed  by  filtration.  In  addition,  the  alumina  in 
some  way  which  is  not  understood  has  a  chemical  attraction 
for  dissolved  organic  matters,  and  the  chemical  purification  and 
removal  of  color  with  the  use  of  alum  may  be  more  complete  at 
very  high  rates  than  would  be  possible  at  any  rate,  however  low, 
with  simple  filtration. 

The  use  of  alum  is  neither  new  nor  peculiar  to  mechanical  fil- 


no 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


ters.  As  early  as  1831  D'Arcet  published  in  the  "  Annales  d'hy- 
giene  publique  "  *  an  account  of  the  purification  of  Nile  water  in 
Egypt  by  adding  alum  to  the  water,  and  afterward  filtering  it 
through  small  household  filters.  More  recently  alum  has  been 
repeatedly  used  in  connection  with  slow  filters,  particularly  at 
Leeuwarden,  Groningen,  and  Schiedam  in  Holland,  where  the 
river  waters  used  for  public  supplies  are  colored  by  peaty  matter 
which  cannot  be  removed  by  simple  filtration. 

Its  use  has,  however,  generally  been  abandoned,  or  at  least 
restricted  to  times  when  the  raw  water  is  unusually  bad,  either 
on  account  of  its  cost,  or  because  it  was  unnecessary,  carefully 
conducted  filtration  without  it  yielding  as  good  an  effluent  as 
was  necessary,  or  because  injurious  effects  followed  its  use. 
Antwerp  furnished  a  striking  case  of  the  latter  in  1893.  During 
extremely  dry  weather  the  much  polluted  river  from  which  the 
water  is  drawn  became  extremely  foul,  and  with  the  small  filter- 
ing area  available  (since  then  increased)  alum  was  resorted  to 
to  secure  a  better  effluent,  especially  as  there  was  some  fear  of 
cholera.  The  water  left  the  filter  quite  clear,  and  free  from 
color,  undecomposed  alum,  or  iron,  and  with  a  neutral  reaction ; 
but  it  apparently  contained  at  least  a  trace  of  free  acid,  for  after 
passing  through  some  miles  of  pipe  on  its  way  to  the  town,  it 
contained  a  considerable  quantity  of  iron  which  it  had  taken 
from  the  pipe,  and  in  addition  it  was  objectionable  in  its  appear- 
ance and  quite  unsuitable  for  laundry  purposes,  and  caused  much 
complaint  and  annoyance  to  the  water  company. 

But  while  alum  has  been  mainly  discarded  in  European  slow 
filters,  it  has  been  adopted  in  the  American  mechanical  filters, 
and  the  favorable  results  which  are  cited  in  connection  with 
them  are  usually  obtained  by  its  use. 

The  commonest  objections  to  the  use  of  alum  are,  that  it  is 
liable  to  leave  the  water  acid,  giving  rise  to  troubles  like  those 
mentioned  above ;  but  this  depends  entirely  upon  the  alkalinity 

*  Translation  in  German  in  Dingler's  Poly  technical  Journal,  1832,  386. 


OTHER   METHODS   OF  FILTRATION.  Ill 

of  the  raw  water  and  the  proportion  of  alum  used,  and  trouble 
of  this  sort  could  probably  be  satisfactorily  corrected  by  adding 
a  small  quantity  of  soda  to  the  water;  and,  second,  that  the  alum 
remaining  in  the  effluent  is  injurious  to  health.  It  is  frequently 
claimed  that  the  decomposition  of  the  alum  is  complete,  and  that 
none  of  the  alumina  goes  into  the  effluent ;  but  the  most  care- 
ful investigation  seems  to  show  that  with  the  rates  of  filtration 
necessarily  employed  with  this  system  a  certain  although  ex- 
tremely small  quantity  of  alum  or  alumina  invariably  remains  in 
the  effluent.  Although  alum  in  large  quantity  is  undoubtedly 
injurious  to  health,  it  is  neither  a  violent  nor  a  cumulative 
poison;  and  the  proposition  that  one  part  of  alumina  in  a  million 
parts  of  water  is  injurious  to  health  must  be  regarded  as  conjec- 
ture rather  than  as  a  matter  of  proof,  or  even  of  probability. 

In  regard  to  the  bacterial  efficiency  of  mechanical  filters  with 
alum,  we  shall  be  fairly  safe  in  concluding  from  analogy  that 
low  (comparative)  rates  are  safer  than  higher  rates,  that  fine  sand 
and  in  a  thick  layer  will  be  safer  than  the  reverse  conditions. 
Experiments  have  also  shown  that  the  amount  of  alum  used 
affects  the  result,  and  that  the  same  results  cannot  be  obtained 
with  a  small  dose  as  with  a  larger  quantity.  We  can  also  assume 
that  the  bacterial  efficiency  will  increase  as  the  filter  becomes 
clogged,  and  that  much  poorer  results  will  be  obtained  immedi- 
ately after  washing  than  at  other  times.  The  statements  of  tests 
frequently  given  out  which  do  not  take  these  important  factors 
into  consideration  can  be  safely  neglected,  as  throwing  no  light 
upon  the  general  problem.  It  is  not  a  difficult  matter  for  an 
intelligent  person  by  skilful  manipulation  to  produce  a  limited 
quantity  of  good  effluent  from  almost  any  filter  within  reason, 
and  the  fact  that  a  good  effluent  is  so  produced  is  of  no  interest 
whatever.  Such  results  only  have  value  when  they  are  known 
to  represent  the  total  or  some  definite  part  of  a  filter's  work 
when  running  under  known  conditions  at  least  approximating 
those  of  actual  practice. 


112  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

A  serious  objection  to  the  use  of  rapid  filters  with  alum  for 
polluted  waters  is  the  possibility  of  poor  work  resulting  from  the 
failure  at  some  time  to  apply  the  alum.  The  success  of  the 
process  from  a  sanitary  standpoint  depends  absolutely  upon  the 
alum  being  applied  to  all  the  water  and  in  the  proper  proportion. 
The  filters  will  probably  run  day  and  night  and  every  day  in  the 
year.  The  failure  to  apply  alum,  say  for  a  few  minutes  in  the 
.middle  of  the  night,  means  that  for  a  time  unpurified  water  will 
be  delivered.  The  first  water  filtered  after  cleaning  also  is  not 
free  from  bacteria,  and  must  be  allowed  to  go  to  waste.  Some- 
time, when  there  is  a  scarcity  of  water  or  through  carelessness, 
the  first  filtrate  may  be  turned  into  the  town  supply  with 
disastrous  results. 

How  much  damage  may  be  caused  by  such  accidents  it  is  im- 
possible to  say  ;  but  after  giving  all  possible  credit  to  automatic 
machinery  and  faithful  attendance,  it  must  be  said  that  they  are 
much  more  likely  to  occur  with  the  rapid  mechanical  filters  than 
with  slow  sand-filters.  With  the  latter,  the  various  conduits 
can  be  so  arranged  that  it  is  practically  impossible  for  a  negli- 
gent employe  to  turn  raw  water  into  the  pure-water  conduit;4 
and  even  if  he  deliberately  attempted  to  do  so,  it  would  be 
necessary  for  him  to  dig  up  the  sand  or  make  some  correspond- 
ing violent  change,  which  could  not  fail  to  be  detected.  The 
water  which  has  actually  penetrated  the  sand  at  the  rates  at 
which  it  will  be  possible  for  it  to  pass  through  the  outlet  open- 
ings will  be  sure  under  all  conditions  to  be  at  least  reasonably 
well  purified  even  if  the  watchman  stupidly  opens  the  gates 
wide  when  a  filter  is  first  put  in  service,  and  such  an  error 
would  be  instantly  and  unmistakably  apparent  to  any  one 
familiar  with  the  normal  working  of  the  filters.  With  a  mechani- 
cal filter,  on  the  other  hand,  it  is  easy  to  imagine  that  the  alum 
solution  might  be  exhausted  without  attracting  attention,  or  that 
a  lazy  attendant  might  fail  to  replace  it  promptly. 

There  are  two  things  which  should  be  clearly  shown  in  re- 


OTHER  METHODS   OF  FILTRATION. 

gard  to  a  mechanical  filter  before  allowing  its  adoption  for  a 
polluted  city  supply :  first,  that  it  is  capable  of  removing  the  bac- 
teria under  normal  conditions  of  operation ;  and,  second,  that 
the  mechanical  arrangements  are  so  perfect  that  the  result 
will  not  be  dependent  upon  the  attendant  to  such  an  extent  that 
mere  negligence,  perhaps  during  the  night  when  detection  is 
improbable,  may  subject  the  city  to  all  the  bad  results  of  polluted 
water.  We  can  wait  until  these  questions  are  settled  in  the 
affirmative  before  considering  the  question  as  to  the  efficiency 
and  cost  of  mechanical  as  compared  to  slow  filtration  without 
chemicals. 

THE   USE  OF  PRECIPITATED   ALUMINA. 

I  do  not  know  that  this  process  has  ever  gone  beyond  the 
experimental  stage,  but  its  plausibility,  and  the  fact  that  some 
little  information  has  been  collected  in  regard  to  it,  require  its 
mention.  Instead  of  adding  the  alum  solution  to  the  water  to 
decompose  in  direct  contact  with  the  matters  it  is  to  affect,  the 
alumina  of  the  solution  is  previously  precipitated  with  soda, 
washed  free  or  nearly  free  from  the  resulting  sulphate  of  soda, 
and  the  flocculent  precipitate  then  added  to  the  water.  By  this 
procedure  both  the  acidification  of  the  effluent  and  the  possibil- 
ity of  the  passage  of  undecomposed  alum  are  entirely  avoided  ; 
but,  on  the  other  hand,  the  power  of  the  precipitated  alumina 
is  very  much  less  than  that  of  a  corresponding  quantity  of  alum. 
Experiments  made  by  this  process  have  not  shown  good  bac- 
terial efficiency  at  the  rates  of  filtration  followed  in  mechanical 
filters. 

THE   USE  OF  OTHER  SALTS. 

Ferric  salts  can  be  used  in  place  of  alum,  but  are  open  to  the 
same  objections  and  I  do  not  know  of  their  having  been  exten- 
sively employed.  Cuprous  chloride  has  been  suggested  by 


H4  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

Krohnke,*  who  obtained  very  satisfactory  bacterial  results  in 
some  experiments  made  at  Hamburg ;  but,  if  only  for  senti- 
mental reasons,  the  process  is  not  likely  to  be  widely  adopted. 

THE   USE   OF    METALLIC    IRON. 

The  use  of  metallic  iron  for  water  purification  in  connection 
with  a  moderately  slow  filtration  through  filters  of  the  usual 
form  is  known  as  Anderson's  process  (patented),  and  has  been 
used  at  Antwerp  and  elsewhere  on  a  large  scale,  and  has  been 
experimentally  examined  at  a  number  of  other  places. 

The  process  consists  in  agitating  the  water  in  contact  with 
metallic  iron,  a  portion  of  which  is  taken  into  solution  as  ferrous 
carbonate.  Upon  subsequent  aeration  this  is  supposed  to  be- 
come oxidized  and  precipitate  out  as  ferric  hydrate,  with  all  the 
good  and  none  of  the  bad  effects  of  alum.  The  precipitate  is 
partially  removed  by  sedimentation,  while  filtration  completes 
the  process.  The  process  is  theoretically  admirable,  and  in  an 
experimental  way  upon  a  very  small  scale  often  gives  most  satis- 
factory results,  muddy  waters  very  difficult  of  filtration  and 
colored  peaty  waters  yielding  promptly  clean  and  colorless 
effluents.  I  have  often  personally  made  these  experiments,  and 
can  vouch  for  the  results,  although  failure  repeatedly  occurs 
under  conditions  apparently  identical  with  those  which  at  other 
times  succeed  brilliantly. 

In  applying  the  process  on  a  larger  scale,  however,  with  peaty 
waters  at  least,  it  seems  impossible  to  get  enough  iron  to  go  into 
solution  in  the  time  which  can  be  allowed,  and  the  small  quan- 
tity which  is  taken  up  either  remains  in  solution  or  else  slowly 
and  incompletely  precipitates  out,  without  the  good  effects  which 
follow  the  sudden  and  complete  precipitation  of  a  larger  quan- 
tity, and  in  this  case  the  color  is  seldom  reduced,  and  may  even 
be  increased  above  the  color  of  the  raw  water  by  the  iron  re- 
maining in  solution. 

Journal  fur  Gas-  und  Wasservcrsorgung,  1883,  p.  513. 


OTHER   METHODS   OF  FILTRATION. 

The  ingenuity  of  those  who  have  studied  the  process  has  not 
yet  found  any  adequate  means  of  avoiding  these  important  prac- 
tical objections  ;  and  even  at  Antwerp  a  great  extension  of  the 
filtering  area,  as  well  as  the  use  of  alum  at  times  of  unusual  pollu- 
tion, is  good  evidence  that  simple  filtration,  in  distinction  from 
the  effect  of  the  iron,  is  relied  upon  much  more  than  formerly. 

At  Dordrecht  also,  where  the  process  has  been  long  in  use,  the 
rate  of  filtration  does  not  exceed  the  ordinary  limits ;  nor  is  the 
result,  so  far  as  I  could  ascertain,  in  any  way  superior  to  that 
obtained  a  few  miles  away,  at  Rotterdam,  by  ordinary  filtration, 
with  substantially  the  same  raw  water. 

The  results  obtained  at  Boulogne-sur-Seine,  near  Paris,  have 
been  closely  watched  by  the  public  chemist  and  bacteriologist  of 
Paris,  and  have  been  very  favorable,  and  a  new  plant  with  a. 
capacity  of  7,900,000  gallons  daily  is  just  being  built  at  Choisy- 
le-Roi,  to  supply  some  of  the  suburbs  of  Paris,  but  even  in  these 
cases  only  moderate  rates  of  filtration  are  employed  which  would 
yield  excellent  effluents  without  the  iron. 

HOUSEHOLD    FILTERS. 

The  subject  of  household  filters  is  a  somewhat  broad  one,  as- 
the  variety  in  these  filters  is  even  greater  than  in  the  larger 
filters,  and  the  range  in  the  results  to  be  expected  from  them  is 
at  least  as  great.  I  shall  only  attempt  to  indicate  here  some  of 
the  leading  points  in  regard  to  them. 

Household  filters  may  be  used  to  remove  mud  or  iron  rust 
from  the  tap  water,  or  to  remove  the  bacteria  in  case  the  latter 
is  sewage-polluted,  or  to  do  both  at  once.  Perhaps  oftener  they 
are  used  simply  because  it  is  believed  to  be  the  proper  thing,  and 
without  any  clear  conception  either  of  the  desired  result  or  the 
way  in  which  it  can  be  accomplished.  I  shall  consider  them 
onl}<  in  their  relations  to  the  removal  of  bacteria,  as  I  credit  the 
people  who  employ  them  with  being  sufficiently  good  judges  of 
their  efficiency  in  removing  visible  sediment. 


Il6  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

In  the  first  place,  as  a  general  rule,  which  has  very  few  if 
any  exceptions,  we  may  say  that  all  small  filters  which  allow  a 
good  stream  of  water  to  pass  do  not  remove  the  bacteria.  The 
reason  for  this  is  simply  that  a  material  open  enough  to  allow 
water  to  pass  through  it  rapidly  is  not  fine  enough  to  stop  such 
small  bodies  as  the  bacteria.  The  filters  which  are  so  often  sold 
as  "  germ-proof,"  consisting  of  sand,  animal  charcoal,  wire-cloth, 
filter-paper,  etc.,  do  not  afford  protection  against  any  unhealthy 
qualities  which  there  may  be  in  the  raw  water.  Animal  charcoal 
removes  color  without  retaining  the  far  more  objectionable 
bacteria. 

The  other  household  filters  have  filtering  materials  of  much 
finer  grain,  unglazed  porcelain  and  natural  sandstone  being  the 
most  prominent  materials,  while  infusorial  earth  is  also  used. 
The  smaller  sizes  of  these  filters  allow  water  to  pass  only  drop  by 
drop,  and  when  a  fair  stream  passes  them  the  filters  have  consid- 
erable filtering  area  (as  a  series  of  filter-tubes  connected  together). 
On  account  of  their  slow  action,  filters  of  this  class  are,  as  a  rule, 
provided  with  storage  reservoirs  so  that  filtered  water  to  the 
capacity  of  the  reservoir  can  be  drawn  rapidly  (provided  the 
calls  do  not  come  too  often).  Some  of  these  filters  are  nearly 
germ-proof,  and  are  comparable  in  their  efficiency  to  large  sand- 
filters.  There  is  no  sharp  line  between  the  filters  which  stop 
and  which  do  not  stop  the  bacteria  ;  but  in  general  the  rule  that 
a  filter  which  works  rapidly  in  proportion  to  its  size  does  not  do 
so,  and  vice  versa,  will  be  found  correct. 

In  thinking  of  the  efficiency  of  household  filters  we  must  dis- 
tinguish between  the  filter  carefully  prepared  for  an  award  at  an 
exhibition  and  the  filter  of  the  same  kind  doing  its  average 
daily  work  in  the  kitchen.  If  we  could  be  sure  in  the  latter  case 
that  an  unbroken  layer  of  fine  sandstone  or  porcelain  was  always 
between  ourselves  and  the  raw  tap-water  we  could  feel  compara- 
tively safe.  The  manufacturers  ot  the  filters  claim  that  leaky 
joints,  cracked  tubes,  etc.,  are  impossible  ;  but  I  would  urge  upon 


OTHER   METHODS   OF  FILTRATION.  1 1/ 

the  people  using  water  filtered  in  this  way  that  they  personally 
assure  themselves  that  this  is  actually  the  case  with  their  own 
filters,  for  in  case  any  such  accident  should  happen  the  conse- 
quences might  be  most  unpleasant.  The  increased  yield  of  a 
filter  due  to  a  leaky  joint  is  sure  not  to  decrease  it  in  favor  with 
the  cook,  who  is  probably  quite  out  of  patience  with  it  because 
it  works  so  slowly,  that  is,  in  case  it  is  good  for  anything. 

The  operation  of  household  filters  is  necessarily,  with  rare  ex- 
ceptions, left  to  the  kitchen-girl  and  luck.  Scientific  supervision 
is  practically  impossible.  With  a  large  filter,  on  the  other  hand, 
concentrating  all  the  filters  for  the  city  at  a  single  point,  a  com- 
petent man  can  be  employed  to  run  them  in  the  best-known 
way  ;  and  if  desired,  and  as  is  actually  done  in  very  many  places, 
an  entirely  independent  bacteriologist  can  be  employed  to  deter- 
mine the  efficiency  of  filtration.  With  the  methods  of  examina- 
tiun  now  available,  and  a  little  care  in  selecting  the  times  and 
places  of  collecting  the  samples,  it  is  quite  impossible  for  a  filter- 
superintendent  to  deliver  a  poor  effluent  very  often  or  for  any 
considerable  length  of  time  without  being  caught.  The  safety 
of  properly-conducted  central  filtration  is  thus  infinitely  greater 
than  that  from  even  the  best  household  filters.  Further,  it  may 
be  doubted  whether  an  infected  water  can  be  sent  into  every 
house  in  the  city  to  be  used  for  washing  and  all  the  purposes 
to  which  water  is  put  except  drinking,  without  causing  disease, 
although  less  than  it  would  if  it  were  also  used  for  drinking. 

The  use  of  household  filters  must  be  regarded  as  a  somewhat 
desperate  method  of  avoiding  some  of  the  bad  consequences  of 
a  polluted  water-supply,  and  they  are  adopted  for  the  most  part 
by  citizens  who  in  some  measure  realize  the  dangers  from  bad 
water,  but  who  cannot  persuade  their  fellow-citizens  to  a  more 
thorough  and  adequate  solution  of  the  problem.  Such  citizens, 
by  the  use  of  the  best  filters,  and  by  carefully  watching  their 
action,  or  by  having  their  drinking-water  boiled,  can  avoid  the 
principal  dangers  from  bad  water,  but  their  vigilance  does  not 
protect  their  more  careless  neighbors. 


Il8  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


CHAPTER   IX. 
COST  AND  ADVANTAGES  OF   FILTRATION. 

THE  cost  of  filtration  of  water  by  the  methods  now  used  in 
Europe  if  adopted  in  the  United  States  would  depend  so  largely 
upon  local  conditions  that  any  accurate  general  estimate  is  quite 
impossible.  Nevertheless  a  little  consideration  of  the  subject,  al- 
though not  leading  to  exact  results,  may  be  helpful  as  furnishing  a 
rough  idea  of  the  probable  cost  before  estimates  for  local  conditions 
are  made. 

In  England  not  much  is  to  be  learned  in  regard  to  the  cost  of 
filtration,  because  as  a  rule  no  separate  accounts  are  kept  of  the 
filters.  In  Germany  such  accounts  are  generally  kept,  and  many 
results  are  available.  The  average  cost  of  building  open  filters  with 
all  the  latest  improvements,  with  the  necessary  reservoirs  for  filtered 
water,  sand-washing  apparatus,  etc.,  but  not  including  sedimenta- 
tion-basins or  the  cost  of  land,  is  estimated  by  Lindley,  an  engineer 
of  very  wide  experience,  at  from  $42,000  to  $48,000  per  acre  of 
effective  filtering  area,  while  for  corresponding  covered  filters  he 
estimates  the  cost  at  $63,000  to  $73,000  per  acre. 

The  total  cost  of  filtering  water,  that  is,  the  operating  expenses, 
with  4  per  cent  on  the  capital  invested  for  interest  and  2  per  cent 
for  sinking  funds,  and  10  per  cent  depreciation  on  machinery,  is  at 
Zurich  with  clear  lake-water  only  0.65  cent  per  thousand  gallons, 
but  in  general  is  from  0.7  to  1.3  cents,  of  which  from  one  third  tc 
one  half  consists  of  the  operating  expenses,  and  the  remainder  is  foi 
the  various  charges  for  capital  and  depreciation. 

The  actual  building  costs  of  some  German  and  other  works  hav< 
been  as  follows : 


COST  AND   ADVANTAGES   OF  FILTRATION.  1 19 

Date  of  Cost  per  Acre. 

r    ce'  Construction. 


Covered.  Open. 

Stralau 1874  $62,000             

Tegel,  first  part 1 884  66,000             

"       second"   1887  70,000            

Hamburg 1892-3             67,000* 

Konigsberg,  first  part.. ..  1883                  20,000 

"             second"...  1889  39,000             

Magdeburg.   1876  83,000             

Warsaw 1885  78,000              

Zurich 1885  86,000             

Lawrence 1892-3             27,000 

Nantucket 1892                45,5oo 

Let  us  consider  the  case  of  a  city  in  the  United  States  with 
100,000  inhabitants  now  drawing  its  water  from  a  polluted  lake  or 
river.  The  quantity  of  water  required  is  perhaps  80  or  100  gallons 
per  inhabitant  daily.  Let  us  assume  an  average  consumption  of 
8,000,000  gallons  daily  and  an  ordinary  maximum  of  10,000,000  gal- 
lons. Half  this  quantity  would  be  considered  an  ample  allowance 
in  England,  and  a  still  smaller  quantity  would  be  required  by  Conti- 
nental cities  of  this  size.  Why  do  American  cities  require  so  much 
more  water  than  European  cities  ?  If  there  was  any  reason  to  be- 
lieve that  the  enormous  volumes  of  water  used  in  America  contrib- 
uted to  the  health,  cleanliness,  or  comfort  of  those  who  use  or 
waste  them,  we  should  not  wish  to  reduce  them,  but  many  Euro- 
pean cities  are  clean  beyond  the  dreams  of  an  American  alderman, 
are  bountifully  supplied  with  public  fountains,  and  water  is  supplied 
abundantly  and  at  low  rates  to  all  inhabitants  for  domestic  and 
manufacturing  purposes.  And  still  the  per-capita  consumption  is 
less  than  a  third  of  the  American  requirement.  But  this  is  not  the 
place  for  a  discussion  of  the  quantity  of  water  required ;  the  exist- 
ing condition  must  be  either  met  or  changed. 

*  Includes  cost  of  large  sedimentation-basins,  pumps,  and  conduits,  etc. 


120  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

To  supply  a  maximum  of  10,000,000  gallons  daily,  five  filters 
each  with  an  area  of  one  acre  will  be  ample.  Any  four  of  them  can 
easily  furnish  this  quantity  while  the  fifth  is  out  of  use  for  cleaning 
or  other  cause.  If  the  city  is  north  of  the  line  of  normal  January 
temperature  of  32°  (page  17),  vaulted  filters  will  be  required.  Such 
filters  have  formerly  cost  on  an  average  $68,000  per  acre  including 
everything  in  Germany,  but  the  vaultings  of  these  filters  were  often 
excessively  thick,  and  the  rather  small  size  of  the  single  beds  in- 
creased the  proportion  of  walls,  regulators,  piping,  etc.  Lighter 
vaulting  has  been  used  on  some  new  filters  for  which  I  have  no 
statements  of  cost. 

Some  estimates  recently  made  by  the  author  in  connection 
with  engineers  examining  the  Boston  Metropolitan  Water-supply 
indicate  that  filters  fully  up  to  the  German  standards,  but  with 
beds  of  a  full  acre  each,  and  with  vaulting  substantially  like  that 
successfully  used  on  the  Newton  covered  reservoir,  can  be  built  at 
present  American  prices  for  somewhat  less  than  the  cost  given 
above,  notwithstanding  the  higher  price  paid  for  American  labor. 

Including  the  connection  with  the  (existing)  pumping-station  we 
may  estimate  the  cost  of  our  five  acres  at  $350,000,  with  a  probabil- 
ity that  with  favorable  local  conditions  the  expenditure  would  be 
still  less.  A  greater  number  of  filters  would,  of  course,  be  designed 
to  provide  for  increasing  population,  but  only  so  many  need  be  con- 
structed as  will  meet  the  present  requirement  or  that  of  the  next 
two  or  three  years,  and  additional  filters  can  be  added  at  about  the 
same  proportional  cost  when  they  are  needed. 

The  heaviest  cost  for  operation  will  be  the  cleaning  of  the 
filters.  Estimating  that  50,000,000  gallons  pass  each  filter  between 
scrapings,  the  total  number  of  scrapings  of  one  filter  in  a  year  will 
be  58,  or  about  once  a  month  for  each  of  them,  necessitating  about 
i/OO  days'  labor.  The  labor  for  the  annual  deeper  scraping  and 
replacing  the  sand  and  for  sand-washing  can  be  estimated  at  as 
much  more,  necessitating  the  permanent  employment  of  twelve 
men.  Three  men,  one  for  each  shift,  will  be  kept  always  on  duty 


COST  AND   ADVANTAGES   OF  FILTRATION. 


121 


to  tend  the  gates,  superintend  work,  and  act  as  watchman.  We 
may  then  estimate  the  cost  of  filtration  for  a  plant  of  this  size  as 
follows : 

COST  OF   OPERATION. 

1 2  laborers,  300  days,  at  $2 $7,2OO 

3  gatemen,    365    "    ,,"  $3 3,285 

New  sand  and  other  supplies 1,000 

Superintendence,  bacterial  examination  of  effluents,  and  ex- 
periments      4,000 


Total  cost  of  operation $i 5,485 

Interest  and  sinking  fund  on  $350,000  at  6% 21,000 


Total  cost  of  filtering  365  times  8,000,000  gallons  $36,485 
Equal  to  1.25  cents  per  thousand  gallons. 

It  is  often  thought  that  a  great  economy  would  result  from  fil- 
tering at  a  higher  rate  than  that  used  in  the  above  calculation.  The 
following  table  has  been  prepared  to  show  that  this  is  not  the  case, 


RELATIVE  COST  OF  FILTERING   AT  VARIOUS   RATES. 
AREAS  AND  COSTS   FOR  ONE   MILLION   GALLONS  AVERAGE  YIELD. 


Rate, 

Million 
Gallons 
per  Acre 
Daily. 

Area  Available  for 
Use  at  one  time, 
allowing  2$%  extra 
for  Maximum 
Consumption. 

Reserve 
Area 
for 

Clean- 
ing.* 
Acres. 

Total 
Area  re- 
quired. 

Acres. 

Cost  at 
$66,000 
per  Acre. 

Interest 
and 
Sinking 
Fund,  6*. 

Operat- 
ing Ex- 
penses. 

Total  Cost 
One  Year. 

Rela- 
tive 
Cost. 

0-5 

2.  500  acres 

0.050 

2.550 

$168,000 

$10,100 

$1500 

$11,600 

320 

I.O 

1.250 

1.300 

86,000 

5»I50 

6,650 

184 

i.5 

0.833 

0.883 

58,300 

3.500 

5,000 

I38 

2.0 

0.625 

0.675 

44,500 

2,680 

4,  1  80 

116 

2-5 

0.500 

0.550 

36,300 

2,  1  80 

3,680 

1  02 

2.57 

0.486 

0.536 

35400 

2,120 

3,620 

100 

3 

0.417 

0.467 

30,800 

1,850 

3,350 

93 

4 

0.312 

0.362 

23,900 

1,440 

2,940 

5 

0.250 

0.300 

19,800 

1,190 

2,690 

74 

6 

0.208 

0.258 

17,000 

1,020 

2,520 

70 

7 

0.179 

0.229 

15,100 

910 

2,410 

67 

8 

0.156 

0.206 

13,600 

820 

2,320 

64 

9 

0.139 

0.189 

12,500 

750 

2,250 

62 

10 

o.  125 

0.175 

1  1,  600 

690 

2,190 

6r 

*  Based  upon  calculations  for  a  very  large  plant.     With  daily  quantities  of  less  than 
20,000,000  gallons,  a  larger  allowance  is  necessary. 


122  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

for  even  if  a  rate  twice  as  great  as  that  recommended  were  used, 
the  cost  would  be  74  per  cent  as  great,  and  with  a  rate  four  times 
as  high  the  cost  would  still  be  61  per  cent  of  the  estimate. 


THE  OBJECTS  OF  FILTRATION. 

t   ,' 

1.  Removal    of   Turbidity. — The     earliest    filters    were    con- 
structed largely,  if  not  mainly,  to  remove  the  visible  impurities  from 
muddy  river-waters,  and  this  object  remains  an  important  one  at 
the  present  time.     For  my  part,  I  find  as  much  pleasure  in  a  bright 
glass  of  water  drawn  from  the  city  tap  as  in  a  stately  city  hall ;  and 
a  muddy  fluid   on  the  ta,ble  is  at   least   as   repulsive  as   an   ugly 
building.     I  am   thus  inclined  purely   from    an  aesthetic   point   of 
view  to  consider  the  filtration  of  turbid  public  water-supplies  as 
equally  important  with  the  construction  of  handsome  public  build- 
ings. 

2.  Removal  of  Tastes  and  Odors.— Many  pond  or  reservoir 
waters  which  are  not  muddy,  and  which  are  not  infected  by  sewage, 
and  which  cannot  be  objected  to  on  sanitary  grounds,  are  still  sub- 
ject at  times  to  the  growth  of  low  forms  of  plants  which,  either  in 
their  growth  or  decomposition,  impart  to  the  water  disagreeable 
tastes  and  odors.     In  such  cases  filters  are  sometimes  provided  to 
remove  the  unpleasant  properties  as  well  as  the  organisms  which 
produce  them.     The  filters  for  the  Liverpool  and  Bradford  supplies 
as  well  as  those  for  the  new  supplies  of  Birmingham  and  Middles- 
borough  in  England  are  mainly  useful  in  this  way. 

3.  Removal  of  the  Danger  from   Cholera. — The  reasons  for 
believing  that  cholera  is  caused  by  polluted  water  are  entirely  simi- 
lar to  those  in  the  case  of  typhoid   fever.     It  was  no  accident  that 
the  epidemic  of  cholera  which  caused  the  death  of  3400  persons  fol- 
lowed the  temporary  supply  of  unfiltered  water  by  the  East  London 
Water  Company  in  1866,  while  the  rest  of  London  remained  nearly 
free,  or  that  the  only  serious  outbreak  of  cholera  in  Western  Europe 


COST  AND    ADVANTAGES   OF  FILTRATION.  123 

in  1892  was  at  Hamburg,  which  was  also  the  only  city  in  Germany 
which  used  raw  river-water.  This  latter  caused  the  sickness  of 
20,000  and  the  death  of  over  8000  people  within  a  month,  and  an 
amount  of  suffering  and  financial  loss,  with  the  panics  which  resulted, 
that  cannot  be  estimated,  but  that  exceeded  many  times  the  cost  of 
the  filters  which  have  since  been  put  in  operation.  Hamburg  had 
several  times  before  suffered  severely  from  cholera,  and  the  removal 
of  this  danger  was  a  leading,  although  not  the  sole,  motive  for  the 
construction  of  filters. 

How  little  cities  supplied  with  pure  water  have  to  dread  from 
cholera  is  shown  by  the  experience  of  Altona  and  other  suburbs  of 
Hamburg  with  good  water-supplies,  which  had  but  few  cases  of 
cholera  not  directly  brought  from  the  latter  place,  and  by  the  ex- 
perience of  England,  which  maintained  uninterrupted  commercial 
intercourse  with  the  plague-stricken  city,  absolutely  without  quaran- 
tine, and,  notwithstanding  a  few  cases  which  were  directly  imported, 
the  disease  gained  no  foothold  in  England. 

I  do  not  know  of  a  single  modern  European  instance  where  a 
city  with  a  good  water-supply  not  directly  infected  by  sewage  has 
suffered  severely  from  cholera.  I  shall  leave  to  others  more 
familiar  with  the  facts  the  discussion  of  what  happened  before  the 
introduction  of  modern  sanitary  methods,  as  well  as  of  the  present 
conditions  in  Asia;  although  I  believe  that  in  these  cases  also 
there  is  plenty  of  evidence  as  to  the  part  water  plays  in  the  spread 
of  the  disease. 

A  considerable  proportion  of  the  water-supplies  of  the  cities  of 
the  United  States  are  so  polluted  that  in  case  cholera  should  gain 
a  foothold  upon  our  shores  we  have  no  ground  for  hoping  for  the 
favorable  experience  of  the  English  cities  rather  than  the  plague  of 
Hamburg  in  1892. 

4.  Prevention  of  Typhoid  Fever. — The  most  characteristic 
and  uniform  result  of  direct  pollution  of  public  water-supplies 
is  the  typhoid  fever  which  results  among  the  users  of  the  water. 
In  the  English  and  German  cities  with  almost  uniformly  good 


124  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

drinking-water,  typhoid  fever  is  already  nearly  exterminated,  and  is 
decreasing  from  year  to  year.  American  cities  having  unpolluted 
water-supplies  have  comparatively  few  deaths  from  this  cause, 
although  the  figures  never  go  so  low  as  in  Europe,  perhaps  on 
account  of  the  fresh  cases  which  are  always  coming  in  from  less 
healthy  neighborhoods  in  ever-moving  American  communities.  In 
other  American  cities  the  death-rates  from  typhoid  fever  are  many 
times  what  they  ought  to  be  and  what  they  actually  are  in  other 
cities,  and  the  rates  in  various  places,  and  in  the  same  place  at  dif- 
ferent times  bear  in  general,  a  close  relation  to  the  extent  of  the 
pollution  of  the  drinking-water.  The  power  of  suitable  filtration 
to  protect  a  city  from  typhoid  fever  is  amply  shown  by  the  very  low 
death-rates  from  this  cause  in  London,  Berlin,  Breslau,  and  large 
numbers  of  other  cities  drawing  their  raw  water  from  sources  more 
contaminated  than  those  of  any  but  the  very  worst  American  sup- 
plies. 

It  is  obvious  that  if  the  healthfulness  of  water  is  made  the  main 
argument  for  filtration,  it  will  be  necessary  to  show  a  prospect  of  a 
considerable  benefit  to  be  obtained,  commensurate  with  the  outlay 
involved.  Great  engineering  works  will  not  be  undertaken  to  save 
a  single  life  or  to  prevent  a  dozen  cases  of  fever.  Filtration  must 
be  considered  in  the  same  way  as  a  project  for  the  abolition  of 
grade  crossings  or  any  other  public  danger :  its  cost  must  be 
balanced  against  the  value  of  the  lives  saved. 

In  making  calculations  of  this  sort  the  value  of  a  life  is  com- 
monly taken  at  $5000.  When  we  remember  that  hardly  any  young 
children  and  but  few  aged  people  suffer  from  typhoid  fever  ;  that 
it  is  essentially  a  disease  of  youth  and  early  middle  age,  a  very 
great  majority  of  deaths  being  of  people  between  the  ages  of  15 
and  50  years  ;  and  also  that  it  affects  to  substantially  the  same 
extent  all  classes  of  society,  the  rich  as  well  as  the  poor,  this  does 
not  seem  an  excessive  valuation.  In  Chicago  in  1891,  1997  people 
are  reported  to  have  died  from  typhoid  fever,  representing  at  this 
rate  a  loss  of  $9,985,000.  I  shall  not  take  into  account  the  suffering 


COST  AND   ADVANTAGES   OF  FILTRATION.  12$ 

and  loss  of  time  for  15,000  or  more  other  people  who  were  seriously 
sick  but  did  not  die,  as  the  death-figures  alone  are  ample  for  my 
present  purpose.  The  great  majority  of  those  cases,  I  think  I  can 
safely  say  nine  tenths  of  them,  were  caused  either  directly  or  indi- 
rectly by  the  polluted  lake-water  from  an  inlet  (since  abandoned) 
but  a  little  way  from  the  outlets  of  numerous  sewers  and  of  the 
Chicago  River.  If  it  is  said  that  the  other  tenth  would  have  prob- 
ably occurred  with  ever  so  good  a  water-supply,  the  above  estimate 
does  not  include  the  thousands  of  people  who,  temporarily  stopping 
in  the  city,  contracted  the  disease  and  went  home  to  suffer  or  die, 
nor  the  thousands  of  others  who  had  left  their  homes  to  find  em- 
ployment in  the  city  and  who  returned  when  they  found  themselves 
sick.  The  deaths  which  resulted  in  this  way  raised  the  typhoid 
death-rates  of  the  entire  surrounding  country  and  even  of  remote 
places.  Thus  I  found,  in  investigating  the  water-supply  of  a  small 
city  over  one  hundred  miles  from  Chicago,  that  of  the  apparently 
too  large  number  of  deaths  from  typhoid  fever,  more  than  one  half 
of  the  cases  were  clearly  contracted  in  Chicago,  and  half  of  the 
others  originated  in  another  city  which  used  river-water. 

The  cost  of  filtration  for  the  Chicago  water  supply  cannot 
be  told  without  careful  estimates,  but  it  is  hardly  likely  that  the 
cost  would  reach  the  $10,000,000  loss  from  typhoid  fever  experienced 
in  one  (unusually  bad)  year  alone. 

Taking  the  estimated  cost  of  filtration  for  100,000  people  in 
America  given  above,  it  will  be  seen  that  if  filtration  resulted  in 
saving  seventy  lives  in  a  year,  their  value,  at  $5000  each,  meets  the 
estimated  cost  of  construction,  $350,000;  and  if  only  seven  lives  are 
saved,  their  value  is  sufficient  to  meet  the  operating  expenses  and 
interest  on  the  capital  invested.  We  can  thus  say  roughly  that  fil- 
tration will  prove  a  profitable  investment  when  it  results  in  a  reduc- 
tion of  the  death-rate  by  at  least  7  per  100,000,  and  the  saving  will 
be  sufficient  to  pay  for  the  cost  of  construction  of  the  filters  in  a 
single  year  when  the  reduction  is  at  least  70  per  100,000. 

The  following  is  a  list  of  the  cities  of  50,000  inhabitants  and  up- 


126 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


ward  in  the  United  States,  with  deaths  from  typhoid  fever  and  the 
sources  of  their  water-supplies.  The  deaths  and  populations  are  from 
the  U.  S.  Census  for  1890;  the  sources  of  the  water-supplies,  from 
the  American  Water-Works  Manual  for  the  same  year.  Four  cities 
of  this  size — Grand  Rapids,  Lincoln,  St.  Joseph,  and  Des  Moines — 
are  not  included  in  the  census  returns  of  mortality.  Two  cities 
with  less  than  50,000  inhabitants  with  exceptionally  high  death- 
rates  have  been  included,  and  at  the  foot  of  the  list  are  given  cor- 
responding data  for  some  large  European  cities  for  1893. 
TYPHOID  FEVER  DEATH-RATES  AND  WATER-SUPPLIES  OF  CITIES. 


City. 

Population. 

Deaths  from 
Typhoid 
Fever. 

Water-supply. 

Total. 

Per 

100,000 

living. 

Birmingham... 
i    Denver  

26,178 
106,713 
105,287 

58,313 
238,617 
44,654 
181,830 

54,955 
230,392 

77,696 
163,003 
161,129 
1,046,964 
1,099,850 
65.533 
94,923 
61,431 
133-156 
60,956 

50.395 
76,168 
261,353 
81,388 

53.230 
74,398 
164,738 
298,997 
105,436 
296,908 
64,495 
58,661 
434,439 

69 
232 
192 

77 
304 
54 
181 

54 

200 
64 
134 
122 

770 

794 
47 
67 
43 
92 

42 
34 
49 
164 

5° 
32 
44 
94 
1  66 

57 
I51 
33 
29 

202 

264 
217 
182 
132 
127 
121 
IOO 
98 

87 
82 
82 
76 

74 
72 
72 
7i 
70 
69 
69 
67 
64 

^ 
61 

60 
59 
57 
56 
54 
5i 
5i 
49 
47 

Five  Mile  Creek 
North  Platte  River  and  wells 
Allegheny  River 
Delaware  River 
Allegheny  and  Monongahela  rivers 
Merrimac  River 
Passaic  River                       [Ions  daily 
Artesian  wells  yielding  1,600,000  gal- 
Potomac  River 
Merrimac  River 
Passaic  River 
Ohio  River 
Delaware  and  Schuylkill  rivers 
Lake  Michigan 
South  River 
Hudson  River 
Brandywine  Creek 
Lakes                                          [ervoirs 
Hudson  River  and  impounding  res- 
Los  Angeles  River  and  springs 
Cumberland  River 
Lake  Erie 
James  River                          [reservoir 
Connecticut  River  and  impounding 
Watupa  Lake 
Mississippi  River 
Lobus     Creek,    Lake    Merced,    and 
White  River          [mountain  streams 
Ohio  River 
Artesian  Wells 
Maiden  Creek  and  Springs 
Impounding  reservoir 

2.  Allegheny.  .  .. 
3.  Camden 

4.  Pittsburg  
Lawrence  
5    Newark.  .  .  . 

6.  Charleston.  ... 
7.  Washington... 
8.  Lowell  

9.  Jersey  City.  .  . 
10.  Louisville  
ii.  Philadelphia.. 
12.  Chicago  

13.  Atlanta  

14    Albany.  .  . 

15.  Wilmington.  . 
16.  St.  Paul  

1  7    Trov  .  . 

1  8.  Los  Angeles.. 
19.  Nashville  
20.  Cleveland  
21.  Richmond  
22    Hartford 

23.  Fall  River.... 
24.  Minneapolis.  . 
25.  San  Francisco 
26.  Indianapolis.. 
27.  Cincinnati  
28.  Memphis  . 
29.  Reading        . 

30.  Baltimore  

COS  T  A  ND   ADVANTAGES   OF  FIL  TRA  TION. 


127 


TYPHOID    FEVER    DEATH-RATES    AND    WATER-SUPPLIES    OF   CITIES. 


City. 

Population. 

Deaths  from 
Typhoid 
Fever. 

Water-supply. 

Total. 

Per 

00,000 
iving. 

31    Omaha 

140,452 
88,150 
132,146 
132,716 
133,896 
50,756 
448,477 
8l,434 
70,028 

75.215 
255,664 
204,468 
81,298 
84,655 
78,347 
6l,220 

806,343 

'  88^143 
242,039 
205,876 
55.727 
57,458 

4,306,41  1 
667,883 
2,424,705 
437,892 
222,233 
169,828 
1,714,938 
634,878 

353.551 
308,930 

I.435.93I 

63 
38 

53 
53 
53 

20 
174 
29 
24 

145 

24 
80 

61 

22 
22 
20 

15 

194 

348 

18 

45 
40 

9 
9 

719 
138 
609 

69 

12 

37 
104 

45 
43 
40 

40 
39 
39 
39 
36 
34 
32 
32 

30 
27 
26 
26 

25 
24 
23 

20 
19 
19 

16 
16 

17 

20 

\\ 

5 

2 
II 

5 
7 

Missouri  River 
Surface-water  and  wells 
Pawtuxet  River 
Missouri  River 
Hemlock  and  Candice  lakes 
Ohio  River 
Impounding  reservoirs 
Maumee  River 
Impounding  reservoir 
Mississippi  River 
Impounding   reservoir 
Niagara  River 
Lake  Michigan 
Impounding  reservoir 
Impounding  reservoir 
Passaic  River  (higher  up) 
Wells                                          [ervoirs 
Wells,  ponds,  and  impounding  res- 
Impounding  reservoir 
Impounding  reservoir  and  springs 
Mississippi  River 
Detroit  River 
Impounding  reservoir 
Delaware  River 

Filtered  Thames  and  Lea  rivers  and 
Loch  Katrine                   [£  from  wells 
Spring  water 
Filtered  dune-water 
Filtered  Maas  River 
Filtered  dune-water 
Filtered  Havel  and  Spree  rivers 
Filtered  Elbe  River 
Filtered  Oder  River 
Ground-water 
Spring-water 

32.  Columbus  
33.  Providence.  .  . 
34.  Kansas  City.  . 
35.  Rochester  
36.  Evansville  
37    Boston  

38.  Toledo  

39.  Cambridge  .  .. 
40   St   Louis  

41.  Scranton  
42.  Buffalo  

43.  Milwaukee  
44.  New  Haven. 
45.  Worcester  
46.  Paterson   .   . 

47    Dayton 

48.  Brooklyn.  .  .  . 
49.  New  York  
50.  Syracuse  ..... 
51.  New  Orleans.. 
52.  Detroit  ....  .  . 
c-5.  Lvnn 

54.  Trenton  
London  

Glasgow  
Paris 

Amsterdam  .  . 
Rotterdam  
Hague 

Berlin  

Hamburg  
Breslau  .  .  . 

Dresden  

Vienna  

Any  full  discussion  of  these  data  would  require  intimate  aquaint- 
ances  with  the  various  local  conditions  which  it  is  impossible  to 
take  up  in  detail  here,  but  some  of  the  leading  facts  cannot  fail  to 
be  instructive. 

Each  of  the  places  having  over  100  deaths  per  100,000  from 
typhoid  fever  used  unfiltered  river-water.  Lower  in  the  list,  but 


128  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

still  very  high,  Charleston,  said  to  have  been  supplied  only  from 
artesian  wells,  had  an  excessive  rate;  but  the  reported  water-con- 
sumption is  so  low  as  to  suggest  that  private  wells  or  other  means 
of  supply  were  in  common  use.  Chicago  and  Cleveland  both  drew 
their  water  from  lakes  where  they  were  contaminated  by  their  own 
sewage.  St.  Paul's  supply  came  from  ponds,  of  which  I  do  not 
know  the  character.  With  these  exceptions  all  of  the  22  cities 
with  over  50,000  inhabitants,  at  the  head  of  the  list,  had  unfiltered 
river-water. 

The  cities  supplied  from  impounding  reservoirs  as  a  rule  had 
lower  death  rates  and  are  at  the  lower  end  of  the  list,  together  with 
some  cities  taking  their  water  supplies  from  rivers  or  lakes  at 
points  where  they  were  subject  to  only  smaller  or  more  remote 
infection.  Only  three  of  the  American  cities  in  the  list  were  re- 
ported as  being  supplied  entirely  with  ground-water. 

Let  us  assume  that  with  present  American  general  conditions, 
with  a  good  water-supply,  there  may  be  as  many  as  25  deaths  from 
typhoid  fever  in  100,000,  due  to  cases  resulting  from  the  use  of 
impure  milk,  the  importation  of  cases  from  other  towns,  and  other 
minor  causes.  There  are  eight  American  cities  on  the  list  coming 
within  this  limit,  including  New  York  and  Brooklyn,  and  all  of  the 
European  cities  mentioned  are  within,  and  most  of  them  much 
lower  than,  this  figure.  And  not  all  the  cities  within  the  limit 
have  such  unquestionably  pure  supplies  as  to  indicate  that  this  is 
really  the  minimum. 

We  have  every  reason  to  expect  that  eventually,  when  the 
general  conditions  are  better,  rates  much  lower  than  25  in  100,000 
will  be  obtained.  This  figure  is  here  used  only  as  a  rate  which 
any  American  city  with  a  good  water-supply  might  reasonably 
expect  to  reach  at  once  in  spite  of  the  new  cases  constantly 
brought  in  from  less  healthy  neighborhoods. 

Using  this  as  a  basis,  there  were  in  1890  five  cities,  each  with 
over  50,000  inhabitants  (not  including  the  smaller  cities  of  Lawrence 
and  Birmingham)  and  an  aggregate  population  of  690,760  which 


COST  AND   ADVANTAGES    OF  FILTRATION. 

used  unfiltered  surface-waters,  and  had  so  many  unnecessary  deaths 
from  typhoid  fever  that,  at  $5000  each,  the  saving  due  to  filtration 
would  have  paid  for  the  entire  cost  of  the  filters  in  the  first  year 
they  were  in  use.  Lawrence  has  since  built  a  filter,  and  the  saving 
of  life  there  in  the  first  four  months  that  the  filter  was  in  use,  at 
$5000  per  head,  was  enough  to  pay  for  the  filter  actually  built. 

Following  down  the  list,  there  were  sixteen  other  cities,  with  an 
aggregate  population  of  3,717,560,  where  filtration  would  have  paid 
for  itself  in  two  years  or  less,  and  still  eighteen  others,  with  an 
aggregate  population  of  3,238,617,  where  filtration  would  have 
saved  7  or  more  lives  annually  per  100,000,  and  would  have  more 
than  paid  for  the  interest  and  cost  of  operation  of  filters  upon  the 
basis  given  above. 

The  twelve  cities  lowest  on  the  list,  which  used  surface-waters, 
with  an  aggregrate  population  of  3,675,319,  had  so  little  typhoid 
fever  that,  upon,  the  basis  given,  filtration  would  not  have  paid. 
But  even  in  these  cases,  taking  into  account  the  other  advantages  of 
filtration,  as  well  as  the  fact  that  the  unavoidable  death  rate  from 
typhoid  fever  assumed,  25  per  100,000,  might  actually  be  much 
reduced  by  an  entirely  safe  water-supply,  it  is  by  no  means  certain 
that  filtration  would  not  have  been  advantageous  in  many  of  them. 

It  is  not  my  purpose  to  make  too  close  comparisons  between  the 
various  cities  on  the  list ;  some  of  them  may  have  been  influenced 
by  unusual  local  conditions  in  1890.  Others  have  in  one  way  or 
another  improved  their  water-supplies  since  that  date,  and  there  are 
several  cities  in  which  I  know  the  present  typhoid-fever  death- 
rates  to  be  materially  lower  than  those  of  1890  given  in  the 
table.  On  the  other  hand,  it  is  equally  true  that  a  number  of  cities, 
including  some  of  the  larger  ones,  have  since  had  severe  epidemics 
of  typhoid  fever  which  have  given  very  much  higher  rates  than 
those  for  1890. 

These  fluctuations  would  change  the  order  of  cities  in  the  list 
from  year  to  year  ;  they  would  not  change  the  general  facts,  which 
are  as  true  to-day  as  they  were  in  1890.  Nearly  all  of  the  great 


130  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

cities  of  the  United  States  are  supplied  with  unfiltered  surface- 
waters,  and  a  great  majority  of  the  waters  are  taken  from  rivers  and 
lakes  at  points  where  they  are  polluted  by  sewage.  The  death-rates 
from  typhoid  fever  in  those  cities,  whether  they  are  compared  with 
better  supplied  cities  of  this  country,  or  with  European  cities,  are 
enormously  high. 

Such  rates  were  formerly  common  in  European  cities,  but  they 
have  disappeared  with  better  sanitary  conditions.  The  introduction 
of  filters  has  often  worked  marvellous  changes  in  Europe,  and  in 
Lawrence  the  improvement  in  the  city's  health  with  filtered  water 
was  prompt  and  unquestionable.  There  is  every  reason  to  believe 
that  the  general  introduction  of  better  water  in  American  cities 
will  work  corresponding  revolutions ;  and  looking  at  it  from  a 
merely  money  standpoint,  the  value  of  the  lives  and  the  saving  of 
the  expenses  of  sickness  will  pay  handsomely  when  compared  with 
the  cost  of  good  water. 

WHAT  WATERS   REQUIRE  FILTRATION? 

From  the  nature  of  the  case  a  satisfactory  general  answer  to 
this  question  cannot  be  given,  but  a  few  suggestions  may  be  useful. 

In  the  first  place,  -ground-waters  obviously  do  not  require  filtra- 
tion :  they  have  already  in  most  cases  been  thoroughly  filtered  in 
the  ground  through  which  they  have  passed,  and  in  the  exceptional 
cases,  as,  for  instance,  an  artesian  well  drawing  water  through  fis- 
sures in  a  ledge  from  a  polluted  origin,  a  new  supply  will  generally 
be  chosen  rather  than  to  attempt  to  improve  so  doubtful  a  raw 
material. 

•River-waters  should  be  filtered.  It  cannot  be  asserted  that 
there  are  no  rivers  in  montainous  districts  in  which  the  water  is  at 
once  clear  and  free  from  pollution,  and  suitable  in  its  natural  state 
for  water-supply ;  but  if  so,  they  are  not  common,  least  of  all  in  the 
regions  where  water-supplies  are  usually  required.  The  use  of 
river-waters  in  their  natural  state  or  after  sedimentation  only, 


COST  A ND   ADVANTA GES   OF  FIL TRA  TION.  1 3 1 

drawn  from  such  rivers  as  the  Merrimac,  Hudson,  Potomac,  Dela- 
ware, Schuylkill,  Ohio,  and  Mississippi,  is  a  filthy  as  well  as  an 
unhealthy  practice,  which  ought  to  be  abandoned. 

The  question  is  more  difficult  in  the  case  of  supplies  drawn  from 
lakes  or  storage  reservoirs.  Many  such  supplies  are  grossly  polluted  • 
and  should  be  either  abandoned  or  filtered.  Others  are  subject  to 
algae  growths,  or  are  muddy,  and  would  be  much  improved  by  fil- 
tration. Still  others  are  drawn  either  from  unpolluted  water-sheds, 
or  the  pollution  is  so  greatly  diluted  and  reduced  by  storage  that 
no  known  disadvantage  results  from  their  use. 

In  measuring  the  effects  of  the  pollution  of  water-supplies,  the 
typhoid-fever  death-rate  is  a  most  important  aid.  Not  that  typhoid 
fever  is  the  sole  evil  resulting  from  polluted  water,  but  because  it 
is  also  a  very  useful  index  of  other  evils  for  which  corresponding 
statistics  cannot  be  obtained,  as,  for  instance,  the  causation  of 
diarrhceal  diseases  or  the  danger  from  invasion  by  cholera. 

I  think  we  shall  not  go  far  wrong  at  the  start  to  confine  our 
attention  to  those  cities  where  there  are  over  25  deaths  from 
typhoid  fever  per  100,000  of  population.  This  will  at  once  throw 
out  of  consideration  a  large  number  of  relatively  good  supplies,  in- 
cluding those  of  New  York  and  Brooklyn.  It  is  not  my  idea  that 
none  of  these  supplies  cause  disease.  Many  of  them,  as  for  instance 
that  of  New  York,  are  known  to  receive  sewage,  and  it  is  an  in- 
teresting question  worthy  of  most  careful  study  whether  there  are 
cases  of  sickness  resulting  from  this  pollution.  The  point  that  I 
wish  to  make  now  is  simply  that  in  those  cases  the  death-rate  itself 
is  evidence  that,  with  existing  conditions  of  dilution  and  storage, 
the  resulting  damage  of  which  we  have  knowledge  is  not  great 
enough  to  justify  the  expense  involved  by  filtration. 

In  this  connection  it  should  not  be  forgotten  that,  especially 
with  very  small  watersheds,  there  may  be  a  danger  as  distinct  from 
present  damage  which  requires  consideration.  Thus  a  single  house 
or  groups  of  houses  draining  into  a  supply  may  not  appreciably 
affect  it  for  years,  until  an  outbreak  of  fever  on  the  water-shed 


*32  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

results  in  infecting  the  water  with  the  germs  of  disease  and  in 
an  epidemic  in  the  city  below.  This  danger  decreases  with  in- 
creasing size  of  the  water-shed  and  volume  of  the  water  with  which 
any  such  pollution  would  be  mixed,  and  also  with  the  population 
draining  into  the  water,  as  there  is  a  probability  that  the  amount 
of  infection  continually  added  from  a  considerable  town  will  not 
be  subject  to  as  violent  fluctuation  as  that  from  only  a  few  houses. 

Thus  in  Plymouth,  Pa.,  in  1885,  there  were  1104  cases  of  typhoid 
fever  and  1 14  deaths  among  a  population  of  8000,  as  the  result  of 
the  discharge  of  the  dejecta  from  a  single  typhoid  patient  into  the 
water  of  a  relatively  small  impounding  reservoir.  The  cost  of  this 
epidemic  was  calculated  with  unusual  care.  The  care  of  the  sick 
cost  in  cash  $67,100.17,  and  the  loss  of  wages  for  those  who  recovered 
amounted  to  $30,020.08.  The  1 14  persons  who  died  were  earning 
before  their  sickness  at  the  rate  of  $18,419.52  annually. 

Such  an  outbreak  would  hardly  be  possible  with  the  Croton 
water-shed  of  the  New  York  water-supply,  on  account  of  the  great 
dilution  and  delay  in  the  reservoirs,  but  it  must  be  guarded  against 
in  small  supplies. 

Of  the  cities  having  more  than  25  deaths  per  100,000  from  typhoid 
fever,  some  will  no  doubt  be  found  where  milk  epidemics  or  other 
special  circumstances  were  the  cause ;  but  I  believe  in  a  majority  of 
them,  and  in  nearly  all  cases  where  the  rate  is  year  after  year  con- 
siderably above  that  figure,  the  cause  will  be  found  in  the  water-sup- 
ply. Investigation  should  be  made  of  this  point ;  and  if  the  water 
is  not  at  fault,  the  responsibility  should  be  located.  If  the  water  is 
guilty,  it  should  be  either  purified  or  a  new  supply  obtained. 


WATER-SUPPLY  AND   DISEASE— CONCLUSIONS.  133 


CHAPTER  X. 
WATER-SUPPLY  AND  DISEASE— CONCLUSIONS. 

THE  faeces  from  a  man  contain  on  an  average  perhaps  1,000,000,- 
ooo  bacteria  per  gram,*  most  of  them  being  the  normal  bacilli  of  the 
intestines,  Bacillus  coli  communis.  Assuming  that  a  man  discharges 
200  grams  or  about  7  ounces  of  faeces  daily,  this  would  give 
200,000,000,000  bacteria  discharged  daily  per  person.  The  number 
of  bacteria  actually  found  in  American  sewage  is  usually  higher, 
often  double  this  number  per  person ;  but  there  are  other  sources 
of  bacteria  in  sewage,  and  in  addition  growths  or  the  reverse  may 
take  place  in  the  sewers,  according  to  circumstances. 

This  number  of  bacteria  in  sewage  is  so  enormously  large  that 
the  addition  of  the  sewage  from  a  village  or  city  to  even  a  large  river 
is  capable  of  affecting  its  entire  bacterial  composition.  Thus  taking 
the  population  of  Lowell  in  1892  at  85,000,  and  the  average  daily 
flow  of  the  Merrimac  at  6000  cubic  feet  per  second,  and  assuming 
that  200,000,000,000  bacteria  are  discharged  daily  in  the  sewage  from 
each  person,  they  would  increase  the  number  in  the  river  by  1160 
per  cubic  centimeter,  or  about  300,000  in  an  ordinary  glass  of  water. 
The  average  number  found  in  the  water  eight  miles  below,  at  the 
intake  of  the  Lawrence  water-works,  was  more  than  six  times  as 
great  as  this,  due  in  part  to  the  sewage  of  other  cities  higher  up. 

There  is  every  reason  to  believe  that  the  bulk  of  these  bacteria 

*  This  number  was  the  result  of  numerous  counts  made  from  faeces  from  persons 
suffering  with  typhoid  fever  in  the  Lawrence  City  Hospital  in  1891  and  1892.  Mr.  G. 
W.  Fuller,  now  in  charge  of  the  Lawrence  Experiment  Station,  has  kindly  made  some 
further  investigation  of  faeces  from  healthy  people  in  which  the  numbers  have  been 
considerably  lower,  usually  less  than  200,000,000,  per  gram  and  sometimes  as  low  as 
10,000,000  per  gram. 


134  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

were  harmless  to  the  people  of  Lawrence,  who  drank  them;  but  some 
of  them  were  not.  Faeces  of  people  suffering  from  typhoid  fever 
contain  the  germs  of  that  disease.  What  proportion  of  the  total 
number  of  bacteria  in  such  faeces  are  injurious  is  not  known;  but  as- 
suming that  one  fourth  only  of  the  total  number  are  typhoid 
germs,  and  supposing  the  faeces  of  one  man  to  be  evenly  mixed 
with  the  whole  daily  average  flow  of  the  river,  it  would  put  one 
typhoid  germ  into  every  glass  of  water  at  the  Lawrence  intake,  and 
at  low  water  several  times  as  many  proportionately  would  be  added. 
This  gives  some  conception  of  the  dilution  required  to  make  a 
polluted  water  safe. 

One  often  hears  of  the  growth  of  disease-germs  in  water,  but  as 
far  as  the  northern  United  States  and  Europe  are  concerned  there  is 
no  evidence  whatever  that  this  ever  takes  place.  There  are  harm- 
less forms  of  bacteria  which  are  capable  of  growing  upon  less  food 
than  the  disease-germs  require  and  they  often  multiply  in  badly- 
polluted  waters.  Typhoid-fever  germs  live  for  a  longer  or  shorter 
period,  and  finally  die  without  growth.  The  few  laboratory  exper- 
iments which  have  seemed  to  show  an  increase  of  typhoid  germs  in 
water  have  been  made  under  conditions  so  widely  different  from 
those  of  natural  watercourses  that  they  have  no  value. * 

The  proportionate  number  of  cases  of  typhoid  fever  among  the 
users  of  a  polluted  water  varies  with  the  number  of  typhoid  germs 
in  the  water.  Excessive  pollution  causes  severe  epidemics  or  con- 
tinued high  death-rates  according  as  the  infection  is  continued  or 

*  These  experiments,  so  far  as  they  have  come  to  the  notice  of  the  author,  have 
been  made  with  water  sterilized  by  heating,  usually  in  small  tubes  stoppered  with 
cotton-wool  or  other  organic  matter.  In  this  case  the  water,  no  matter  how  carefully 
purified  in  the  first  place,  becomes  an  infusion  of  organic  matters  capable  of  sup- 
porting bacterial  growths,  and  not  at  all  to  be  compared  to  natural  waters. 

In  experiments  often  repeated  under  my  direction,  carefully  distilled  water  in 
bottles,  most  scrupulously  clean,  with  glass  stoppers,  and  protected  from  dust,  but  not 
sterilized,  has  uniformly  refused  to  support  bacterial  growths  even  when  cautiously 
seeded  at  the  start,  and  the  same  is  usually  true  of  pure  natural  waters.  Some  further 
experiments  showed  hardly  any  bacterial  growth  even  of  the  most  hardy  water  bac- 
teria in  a  solution  i  part  of  peptone  in  1,000,000,000  parts  of  distilled  water,  and 
solutions  ten  times  as  strong  only  gave  moderate  growths. 


WATER-SUPPLY  AND   DISEASE— CONCLUSIONS.  135 

intermittent.  Slight  infection  causes  relatively  few  cases  of  fever. 
Pittsburg  and  Allegheny,  taking  their  water-supplies  from  below  the 
outlets  of  some  of  their  own  sewers,  have  suffered  severely  (103.2 
and  127.4  deaths  from  typhoid  fever  annually  per  100,000,  respec- 
tively, from  1888  to  1892).  Wheeling,  W.  Va.,  with  similar  condi- 
tions in  1890,  was  even  worse,  a  death  rate  of  345  per  100,000  from 
this  cause  being  reported,  while  Albany  had  only  comparatively 
mild  epidemics  from  the  less  directly  and  grossly  polluted  Hudson. 
Lawrence  and  Lowell,  taking  their  water  from  the  Merrimac,  both 
had  for  many  years  continued  excessive  rates,  increasing  gradually 
with  increasing  pollution  ;  and  the  city  having  the  most  polluted 
source  had  the  higher  rate. 

In  Berlin  and  Altona,  in  winter,  with  open  filters,  epidemics  of 
typhoid  fever  followed  decreased  efficiency  of  filtration,  but  the 
epidemics  were  often  so  mild  that  they  would  have  entirely  escaped 
observation  under  present  American  conditions.  Chicago  has  for 
years  suffered  from  typhoid  fever,  and  the  rate  has  fluctuated,  as 
far  as  reliable  information  can  be  obtained  with  the  fluctuations  in 
the  pollution  of  the  lake  water.  An  unusual  discharge  of  the 
Chicago  River  results  in  a  higher  death-rate.  Abandoning  the 
shore  inlet  near  the  mouth  of  the  Chicago  River  in  1892,  resulted 
in  the  following  year  in  a  reduction  of  60  per  cent  in  the  typhoid 
fever  death-rate.*  This  reduction  shows,  not  that  the  present  in- 
takes are  safe,  but  simply  that  they  are  less  polluted  than  the  old 
ones  to  an  extent  measured  by  the  reduction  in  the  death-rate. 

It  is  not  supposed  that  in  an  epidemic  of  typhoid  fever  caused  by 
polluted  water  every  single  person  contracts  the  disease  directly  by 
drinking  the  water.  On  the  contrary,  typhoid  fever  is  often  com- 
municated in  other  ways.  If  we  have  in  the  first  place  a  thousand 
cases  in  a  city  caused  directly  by  the  water,  they  will  be  followed 
by  a  large  number  of  other  cases  resulting  directly  from  the  pres- 

*  The  Water-supply  of  Chicago  :  Its  Source  and  Sanitary  Aspects.  By  Arthur  R. 
Reynolds,  M.D.,  Commissioner  of  Health  of  Chicago,  and  Allen  Hazen.  American 
Public  Health  Association,  1893.  Page  146. 


136  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

ence  in  the  city  of  the  first  thousand  cases.  The  conditions  favor- 
ing  this  spread  may  vary  in  different  wards,  resulting  in  considerable 
local  variations  in  the  death-rates.  Some  persons  also  will  suffer 
who  did  not  drink  any  tap-water.  These  facts,  always  noted  in 
epidemics,  afford  no  ground  for  refusing  to  believe,  in  the  presence 
of  direct  evidence,  that  the  water  was  the  cause  of  the  fever.  These 
additional  cases  are  the  indirect  if  not  the  direct  result  of  the  water. 
The  broad  fact  that  cities  with  polluted  water-supplies  as  a  rule 
have  high  typhoid-fever  death-rates,  and  cities  with  good  water- 
supplies  do  not  (except  in  the  occasional  cases  of  milk  epidemics, 
or  where  they  are  overrun  by  cases  contracted  in  neighboring 
cities  with  bad  water,  as  is  the  case  with  some  of  Chicago's  suburbs), 
is  at  once  the  best  evidence  of  the  damage  from  bad  water  and 
measure  of  its  extent. 

The  conditions  which  remove  or  destroy  the  sewage  bacteria  in 
a  water  tend  to  make  it  safe.  The  most  important  of  them  are : 
(i)  dilution  (2)  time,  allowing  the  bacteria  to  die  (sunlight  may  aid 
in  this  process,  although  effective  sunshine  cannot  reach  the  lower 
layer  of  turbid  waters  or  through  ice)  ;  (3)  sedimentation,  allowing 
them  to  go  to  the  bottom,  where  they  eventually  die  ;  and  (4)  natu- 
ral or  artificial  filtration.  In  rivers,  distance  is  mainly  useful  in 
affording  time,  and  also,  under  some  conditions,  in  allowing  oppor- 
tunities for  sedimentation.  Thus  a  distance  of  500  miles  requires 
a  week  for  water  travelling  three  miles  an  hour  to  pass,  and  will 
allow  very  important  changes  to  take  place.  The  old  theory  that 
water  purifies  itself  in  running  a  certain  distance  has  no  adequate 
foundation  as  far  as  bacteria  are  concerned.  Some  purification 
takes  place  with  the  time  involved  in  the  passage,  but  its  extent  has 
been  greatly  overestimated. 

The  time  required  for  the  bacteria  to  die  simply  from  natural 
causes  is  considerable  ;  certainly  not  less  than  three  or  four  weeks 
can  be  depended  upon  with  any  confidence.  In  storage  reservoirs 
this  action  is  often  considerable,  and  it  is  for  this  reason  that 
American  water-supplies  from  large  storage  reservoirs  are,  as  a  rule, 


WATER-SUPPLY  AND   DISEASE— CONCLUSIONS.  137 

much  more  healthy  than  those  drawn  from  rivers  or  polluted  lakes, 
even  when  the  sources  of  the  former  are  somewhat  polluted.  The 
water-supplies  of  New  York  and  Boston  may  be  cited  as  examples. 
In  many  other  water-works  operations  the  entire  time  from  the 
pollution  to  the  consumption  of  the  water  is  but  a  few  days  or  even 
less,  and  time  does  not  materially  improve  water  in  this  period. 

Sedimentation  removes  bacteria  only  slowly,  as  might  be  ex- 
pected from  their  exceedingly  small  size  ;  and  in  addition  their  spe- 
cific gravity  probably  is  but  slightly  greater  than  that  of  water.  The 
Lawrence  reservoir,  holding  from  10  to  14  days'  supply,  effected, 
by  the  combined  effect  of  time  and  sedimentation,  a  reduction  of  90 
per  cent  of  the  bacteria  in  the  raw  water.  In  spite  of  this  the  city 
suffered  severely  and  continuously  from  fever.  It  would  probably 
have  suffered  even  more,  however,  had  it  not  been  for  this  reduc- 
tion. Nothing  is  known  of  the  removal  of  bacteria  by  sedimenta- 
tion from  flowing  rivers,  but,  considering  the  slowness  with  which 
the  process  takes  place  in  standing  water,  it  is  evident  that  we  can- 
not hope  for  very  much  in  streams,  and  especially  rapid  streams, 
where  the  opportunities  for  sedimentation  are  still  less  favorable. 

Filtration  as  practiced  in  Europe  removes  promptly  and  cer- 
tainly a  very  large  proportion  of  the  bacteria — probably,  under  all 
proper  conditions,  over  99  per  cent,  and  is  thus  much  more  effective 
in  purification  than  even  weeks  of  storage  or  long  flows  in  rivers. 
The  places  using  filtered  water  have,  in  general,  extremely  low 
death-rates  from  typhoid  fever.  The  fever  which  has  occurred 
at  a  few  places  drawing  their  raw  water  from  greatly  polluted 
sources  has  resulted  from  improper  conditions  which  can  be 
avoided,  and  affords  no  ground  for  doubt  of  the  efficiency  of  prop- 
erly conducted  filtration. 

The  same  confidence  cannot  be  expressed  in  regard  to  the 
mechanical  filters  often  used  in  America  for  filtering  at  much 
higher  rates  than  those  used  in  European  filters.  There  is  good 
evidence  that  so  far  as  they  do  not  use  alum  they  cannot  even 
approximate  to  the  result  obtained  by  slow  sand  filtration.  With 


138  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

the  use  of  alum  in  sufficient  quantity  better  bacterial  results  are 
obtained,  but  the  evidence  is  not  conclusive  either  that  a  satisfac- 
tory purification  can  or  cannot  be  obtained  with  them.  The  use  of 
such  filters  will  not,  under  any  circumstances,  add  infection  to  the 
water,  and  will  undoubtedly  remove  some  of  the  bacteria.  Between 
no  filtration  and  mechanical  filtration  of  an  infected  water  I  should 
by  all  means  prefer  the  latter.  I  should,  however,  insist  upon 
thorough  and  continued  chemical  and  bacterial  examinations  of  the 
effluent.  If  it  was  found  that  bacteria  were  passing  too  freely,  the 
examinations  would  show  the  danger.  And  in  case  of  an  epidemic 
of  typhoid  fever  or  cholera  the  experience  gained,  although  dear, 
would  be  extremely  valuable  and  conclusive. 

The  main  point  is  that  disease-germs  shall  not  be  present  in  our 
drinking-water.  If  they  can  be  kept  out  in  the  first  place  at  rea- 
sonable expense,  that  is  the  thing  to  do.  Innocence  is  better  than 
repentance.  If  they  cannot  be  kept  out,  we  must  take  them  out 
afterwards  ;  it  does  not  matter  much  how  this  is  done,  so  long  as 
the  work  is  thorough.  Sedimentation  and  storage  may  accomplish 
much,  but  their  action  is  too  slow  and  often  uncertain.  Filtration 
properly  carried  out  removes  bacteria  promptly  and  thoroughly 
and  at  a  reasonable  expense. 


APPENDICES. 


APPENDIX  I. 

RULES  OF  THE  GERMAN  GOVERNMENT  IN  REGARD  TO  THE 
FILTRATION  OF  SURFACE-WATERS  USED  FOR  PUBLIC 
WATER-SUPPLIES. 

Rules  somewhat  similar  to  those  of  which  a  translation  is 
given  below  were  first  issued  by  the  Imperial  Board  of  Health  in 
1892.  These  rules  were  regarded  as  unnecessarily  rigid,  and  a 
petition  was  presented  to  the  government  signed  by  37  water- 
works engineers  and  directors  requesting  a  revision.*  As  a  result 
a  conference  was  organized  consisting  of  14  members.f  Kohler 
presided,  and  Koch,  Gaffsky,  Werner,  Gunther,  and  Reincke  repre- 
sented the  Imperial  Board  of  Health.  The  bacteriologists  were 
represented  by  Flugge,  Wolffhugel,  and  Frankel,  while  Beer, 
Fischer,  Lindley,  Meyer,  and  Piefke  were  the  engineer  members. 

This  conference  prepared  the  17  articles  given  below  in  the  first 
days  of  January,  1894.  A  little  later  the  first  16  articles  were 
issued  to  all  German  local  authorities,  signed  by  Bosse,  minister  of 
the  "  Geistlichen,"  and  Haase,  minister  of  the  interior,  and  they  are 
considered  as  binding  upon  all  water-works  using  surface-water. 
The  bacterial  examinations  were  commenced  April  I,  1894,  by  most 
of  the  cities  which  had  not  previously  had  them. 

*  Journal  fur  Gas-  u.  Wasserversorgung,  1893,  694. 
\  Journal fiir  Gas-  u.  Wasserversorgung,  1894,  185. 

139 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

Although  the  articles  do  not  deal  with  rate  of  filtration,  or  the 
precautions  against  snow  and  ice,  they  have  a  very  great  interest 
both  because  they  are  an  official  expression,  and  on  account  of  the 
personal  standing  of  the  men  who  prepared  them. 

§  I.  In  judging  of  the  quality  of  a  filtered  surface-water  the  fol- 
lowing points  should  be  especially  observed  : 

a.  The   operation  of    a   filter   is  to  be  regarded  as  satisfactory 
when  the  filtrate  contains  the  smallest  possible  number  of  bacteriar 
not  exceeding  the  number  which  practical  experience  has  shown  to 
be   attainable  with   good  filtration   at  the  works  in   question.     In 
those  cases  where  there  are  no  previous  records  showing  the  possi- 
bilities  of  the  works   and  the  influence   of  the   local   conditions, 
especially  the  character  of  the  raw  water,  and  until  such  informa- 
tion is  obtained,  it  is  to  be  taken  as  the  rule  that  a  satisfactory 
filtration  will  never  yield  an  effluent   with   more  than  about  100 
bacteria  per  cubic  centimeter. 

b.  The  filtrate  must  be  as  clear  as  possible,  and,  in  regard  to 
color,  taste,  temperature,  and  chemical   composition,  must  be  no 
worse  than  the  raw  water. 

§  2.  To  allow  a  complete  and  constant  control  of  the  bacterial 
efficiency  of  filtration,  the  filtrate  from  each  single  filter  must  be 
examined  daily.  Any  sudden  increase  in  the  number  of  bacteria 
should  cause  a  suspicion  of  some  unusual  disturbance  in  the  filter, 
and  should  make  the  superintendent  more  attentive  to  the  possible 
causes  of  it. 

§  3.  Filters  must  be  so  constructed  that  samples  of  the  effluent 
from  any  one  of  them  can  be  taken  at  any  desired  time  for  the 
bacteriological  examination  mentioned  in  §  I. 

§  4.  In  order  to  secure  uniformity  of  method,  the  following  is 
recommended  as  the  standard  method  for  bacterial  examination : 

The  nutrient  medium  consists  of  10  per  cent  meat  extract  gel; 
tine  with  peptone,   10  cc.  of  which  is  used   for  each  experiment. 
Two  samples  of  the  water  under  examination  are  to  be  taken,  one 


APPENDIX  I.  141 

of  I  cc.  and  one  of  £  cc.  The  gelatine  is  melted  at  a  temperature 
of  30°  to  35°  C.,  and  mixed  with  the  water  as  thoroughly  as  possible 
in  the  test-tube  by  tipping  back  and  forth,  and  is  then  poured 
upon  a  sterile  glass  plate.  The  plates  are  put  under  a  bell-jar 
which  stands  upon  a  piece  of  blotting-paper  saturated  with  water, 
and  in  a  room  in  which  the  temperature  is  about  20°  C. 

The  resulting  colonies  are  counted  after  48  hours,  and  with  the 
aid  of  a  lens. 

If  the  temperature  of  the  room  in  which  the  plates  are  kept  is 
lower  than  the  above,  the  development  of  the  colonies  is  slower, 
and  the  counting  must  be  correspondingly  postponed. 

If  the  number  of  colonies  in  I  cc.  of  the  water  is  greater  than 
about  100,  the  counting  must  be  done  with  the  help  of  the  Wolff- 
hugel's  apparatus. 

§  5.  The  person  entrusted  with  the  carrying-out  of  the  bacterial 
examinations  must  present  a  certificate  that  he  possesses  the  neces- 
sary qualifications,  and  wherever  possible  he  shall  be  a  regular 
employe  of  the  water-works. 

§  6.  When  the  effluent  from  a  filter  does  not  correspond  to  the 
hygienic  requirements  it  must  not  be  used,  unless  the  cause  of  the 
unsatisfactory  work  has  already  been  removed  during  the  period 
covered  by  the  bacterial  examinations. 

In  case  a  filter  for  more  than  a  very  short  time  yields  a  poor 
effluent,  it  is  to  be  put  out  of  service  until  the  cause  of  the  trouble 
is  found  and  corrected. 

It  is,  however,  recognized  from  past  experience  that  sometimes 
unavoidable  conditions  (high  water,  etc.)  make  it  impossible,  from  an 
engineering  standpoint,  to  secure  an  effluent  of  the  quality  stated  in 
§  I.  In  such  cases  it  will  be  necessary  to  get  along  with  a  poorer 
quality  of  water ;  but  at  the  same  time,  if  the  conditions  demand  it 
(outbreak  of  an  epidemic,  etc.),  a  suitable  notice  should  be  issued. 

§  7.  Every  single  filter  must  be  so  built  that,  when  an  inferior 
effluent  results,  which  does  not  conform  to  the  requirements,  it  can 
be  disconnected  from  the  pure-water  pipes  and  the  filtrate  allowed 


I42  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

to  be  wasted,  as  mentioned  in  §  6.     This  wasting  should  in  general 
take  place,  so  far  as  the  arrangement  of  the  works  will  permit  it : 

(1)  Immediately  after  scraping  a  filter;  and 

(2)  After  replacing  the  sand  to  the  original  depth. 

The  superintendent  must  himself  judge,  from  previous  ex- 
perience with  the  continual  bacterial  examinations,  whether  it  is 
necessary  to  waste  the  water  after  these  operations,  and,  if  so,  how 
long  a  time  will  probably  elapse  before  the  water  reaches  the 
standard  purity. 

§  8.  The  best  sand-filtration  requires  a  liberal  area  of  filter- 
surface,  allowing  plenty  of  reserve,  to  secure,  under  all  local  con- 
ditions, a  moderate  rate  of  filtration  adapted  to  the  character  of  the 
raw  water. 

§  9.  Every  single  filter  shall  be  independently  regulated,  and  the 
rate  of  filtration,  loss  of  head,  and  character  of  the  effluent  shall  be 
known.  Also  each  filter  shall,  by  itself,  be  capable  of  being  com- 
pletely emptied,  and,  after  scraping,  of  having  filtered  water  intro- 
duced from  below  until  the  sand  is  filled  to  the  surface. 

§  10.  The  velocity  of  filtration  in  each  single  filter  shall  be 
capable  of  being  arranged  to  give  the  most  favorable  results,  and 
shall  be  as  regular  as  possible,  quite  free  from  sudden  changes  or 
interruptions.  On  this  account  reservoirs  must  be  provided  large 
enough  to  balance  the  hourly  fluctuation  in  the  consumption  of 
water. 

§  II.  The  filters  shall  be  so  arranged  that  their  working  shall 
not  be  influenced  by  the  fluctuating  level  of  the  water  in  the  fil- 
tered-water  reservoir  or  pump-well. 

§  12.  The  loss  of  head  shall  not  be  allowed  to  become  so  great 
as  to  cause  a  breaking  through  of  the  upper  layer  on  the  surface  of 
the  filter.  The  limit  to  which  the  loss  of  head  can  be  allowed  to 
go  without  damage  is  to  be  determined  for  each  works  by  bacterial 
examinations. 

§  13.  Filters  shall  be  constructed  throughout  in  such  a  way  as 
to  insure  the  equal  action  of  every  part  of  their  area. 


APPENDIX  I.  143 

§  14.  The  sides  and  bottoms  of  filters  must  be  made  water-tight, 
and  special  pains  must  be  taken  to  avoid  the  danger  of  passages  or 
loose  places  through  which  the  unfiltered  water  on  the  filter  might 
find  its  way  to  the  filtered-water  channels.  To  this  end  special 
pains  should  be  taken  to  make  and  keep  the  ventilators  for  the 
filtered-water  channels  absolutely  tight. 

§  15.  The  thickness  of  the  sand-layer  shall  be  so  great  that 
under  no  circumstances  shall  it  be  reduced  by  scraping  to  less  than 
30  cm.  (—  12  inches),  and  it  is  desirable,  so  far  as  local  conditions 
allow,  to  increase  this  minimum  limit. 

Special  attention  must  be  given  to  the  upper  layer  of  sand,  which 
must  be  arranged  and  continually  kept  in  the  condition  most  favor- 
able for  filtration.  For  this  reason  it  is  desirable  that,  after  a  filter 
has  been  reduced  in  thickness  by  scraping  and  is  about  to  be 
refilled,  the  sand  below  the  surface,  as  far  as  it  is  discolored,  should 
be  removed  before  bringing  on  the  new  sand. 

§  16.  Every  city  in  the  German  empire  using  sand-filtered  water 
is  requested  to  make  a  quarterly  report  of  their  working  results, 
especially  of  the  bacterial  character  of  the  water  before  and  after 
filtration,  to  the  Imperial  Board  of  Health  (Kaiserlichen  Gesund- 
heitsamt),  which  will  keep  itself  in  communication  with  the  com- 
mission chosen  by  the  water-works  engineers  in  regard  to  these 
questions ;  and  it  is  believed  that  after  such  statistical  information  is 
obtained  for  a  period  of  about  two  years  some  farther  judgments 
can  be  reached. 

§  17.  The  question  as  to  the  establishment  of  a  permanent 
inspection  of  public  water-works,  and,  if  so,  under  what  conditions, 
can  be  best  answered  after  the  receipt  of  the  information  indicated 
in  §  16. 


144  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


APPENDIX  II. 

EXTRACTS   FROM    « BERICHT    DES    MEDICINAL-INSPECTORATS 
DES   HAMBURGISCHEN   STAATES   FUR   DAS  JAHR  1892." 

THE  following  are  translations  from  Dr.  Reincke's  most  valuable 
report  upon  the  vital  statistics  of  Hamburg  for  1892.  I  much 
regret  that  I  am  unable  to  reproduce  in  full  the  very  complete  and 
instructive  tables  and  diagrams  which  accompany  the  report. 

Diarrhoea  and  Cholera  Infantum  (page  10).  "  It  is  usually 
assumed  that  the  increase  of  diarrhceal  diseases  in  summer  is  to  be 
explained  by  the  high  temperature,  especially  by  the  action  of  the 
heat  upon  the  principal  food  of  infants — milk.  Our  observations, 
however,  indicate  that  a  deeper  cause  must  be  sought."  (Tables 
and  diagrams  of  deaths  from  cholera  infantum  by  months  for 
Hamburg  and  for  Altona  with  the  mean  temperatures,  1871-1892.) 

"  From  these  it  appears  that  the  highest  monthly  mortality  of 
each  year  in  Hamburg  occurred  7  times  in  July,  13  times  in 
August,  and  3  times  in  September,  and  substantially  the  same  in 
Altona.  If  one  compares  the  corresponding  temperatures,  it  is 
found  that  in  the  three  years  1886,  1891,  and  1892,  with  high 
September  mortalities,  especially  the  first  two  of  them,  had  their 
maximum  temperature  much  earlier,  in  fact  earlier  than  usual. 
Throughout,  the  correspondence  between  deaths  and  temperatures 
is  not  well  marked.  Repeated  high  temperatures  in  May  and 
June  have  never  been  followed  by  a  notable  amount  of  cholera 
infantum,  although  such  periods  have  lasted  for  a  considerable 
time.  For  example,  toward  the  end  of  May,  1892,  for  a  long  time 
the  temperature  was  higher  than  in  the  following  August,  when 
the  cholera  infantum  appeared. 


APPENDIX  II.  145 

"  The  following  observations  are  still  more  interesting.  As  is 
seen  from  the  diagram,  in  addition  to  the  annual  rise  in  summer 
there  is  also  a  smaller  increase  in  the  winter,  which  is  especially 
marked  in  Altona.  In  1892  this  winter  outbreak  was  greater  than 
the  summer  one,  and  nearly  as  great  in  1880  and  in  1888.  The 
tew  years  when  this  winter  increase  was  not  marked,  1876-7,  1877-8, 
1 88 1-2,  1883-4,  were  warm  winters  in  which  the  mean  temperature 
did  not  go  below  the  freezing-point.  It  is  also  to  be  noted  that 
the  time  of  this  winter  outbreak  is  much  more  variable  than  that 
of  the  summer  one.  In  1887  the  greatest  mortality  was  in  Novem- 
ber; in  1889  in  February  ;  in  other  years  in  December  or  January, 
and  in  Altona,  in  1886  and  1888,  in  March,  which  is  sufficient 
evidence  that  it  was  not  the  result  of  Christmas  festivities. 

"  Farther,  the  winter  diarrhoea  of  Hamburg  and  of  Altona  are 
not  parallel  as  is  the  case  in  summer.  In  Hamburg  the  greatest 
mortality  generally  comes  before  New  Year's ;  in  Altona  one  to  two- 
months  later. 

"  In  Bockendahl's  Generalbericht  u'ber  das  offentliche  Gesundheits- 
wesen  der  Provinz  Schleswig-Holstein  fur  das  Jahr  1870,  page  ior 
we  read  :  *  Yet  more  remarkable  was  an  epidemic  of  cholera  infantum 
in  Altona  in  February  which  proved  fatal  to  43  children.  These 
cases  were  distributed  in  every  part  of  the  city,  and  could  not  be 
explained  by  the  health  officer  until  he  ascertained  that  the  water 
company  had  supplied  unfiltered  water  to  the  city.  This  occurred 
for  a  few  days  only  in  January,  and  was  the  only  time  in  the  whole 
year  that  unfiltered  Elbe  water  was  delivered.  However  little 
reason  there  may  be  to  believe  that  there  was  a  connection  between 
these  circumstances,  future  interruptions  of  the  service  of  filtered 
water  should  be  most  critically  watched,  as  only  in  this  way  can 
reliable  conclusions  be  reached.  Without  attempting  to  draw  any 
scientific  conclusions  from  the  fact,  I  cannot  do  less  than  record 
that,  prior  to  the  outbreak  of  cholera  on  August  20,  1871,  unfiltered 
together  with  filtered  water  had  been  supplied  to  the  city  August 
ii  to  1 8.  The  action  of  the  authorities  was  then  justified  when 


146  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

they  forbade  in  future  the  supply  of  unfiltered  water  except  in 
-cases  of  most  urgent  necessity,  as  in  case  of  general  conflagration ; 
and  in  such  a  case,  or  in  case  of  interruption  due  to  broken  pipes, 
that  the  public  should  be  suitably  warned.' 

"  The  author  of  this  paragraph,  Dr.  Kraus,  became  later  the 
health  officer  of  Hamburg,  and  in  an  opinion  written  by  him  in 
1874,  and  now  before  me,  he  most  earnestly  urged  the  adoption  of 
sand-filtration  in  Hamburg,  and  cites  the  above  observations  in 
support  of  his  position.  In  the  annual  report  of  vital  statistics  of 
Hamburg  for  1875  he  says  that  it  is  quite  possible  that  the  addition 
of  unfiltered  Elbe  water  to  milk  is  the  cause  of  the  high  mortality 
from  cholera  infantum,  as  compared  with  London,  and  this  idea 
was  often  afterward  expressed  by  him.  Since  then  so  much  evi- 
dence has  accumulated  that  his  view  may  fairly  be  considered 
proved. 

"  For  the  information  of  readers  not  familiar  with  local  condi- 
tions, a  mention  of  the  sources  of  the  water-supplies  up  to  the  pres- 
ent time  used  by  Hamburg  and  Altona  will  be  useful.  Both  cities 
take  their  entire  water-supplies  from  the  Elbe — Altona  from  a 
point  about  7  miles  below  the  discharge  of  the  sewage  of  both 
cities,  Hamburg  from  about  7  miles  above.  The  raw  water  at 
Altona  is  thus  polluted  by  the  sewage  from  the  population  of  both 
cities,  having  now  together  over  700,000  inhabitants,  and  contains  in 
general  20,000  to  40,000  or  more  bacteria  per  cubic  centimeter. 
The  raw  water  of  Hamburg  has,  however,  according  to  the  time  of 
year  and  tide,  from  200  to  5000,  but  here  also  occasionally  much 
higher  numbers  are  obtained  when  the  ebb  tide  carries  sewage  up 
to  the  intake.  How  often  this  takes  place  is  not  accurately  known, 
but  most  frequently  in  summer  when  the  river  is  low,  more  rarely 
in  winter  and  in  times  of  flood.  Recent  bacterial  examinations 
show  that  it  occurs  much  more  frequently  than  was  formerly  as- 
sumed from  float  experiments.  This  water  is  pumped  directly  to 
the  city  raw,  while  that  for  Altona  is  carefully  filtered. 

"  Years  ago  I  expressed  the  opinion  that  the  repeated  typhoid 


APPENDIX  II.  147 

epidemics  in  Altona  stood  in  direct  connection  with  disturbances  of 
the  action  of  the  filters  by  frost,  which  result  in  the  supply  of 
insufficiently  purified  water.  Wallichs  in  Altona  has  also  come  to 
this  conclusion  as  a  result  of  extended  observation,  and  recently 
Robert  Koch  has  explained  the  little  winter  epidemic  of  cholera  in 
Altona  in  the  same  way,  thus  supporting  our  theory.  When  open 
filters  are  cleaned  in  cold,  frosty  weather  the  bacteria  in  the  water 
are  not  sufficiently  held  back  by  the  filters.  Such  disturbances  of 
filtration  not  only  preceded  the  explosive  epidemics  of  typhoid 
fever  of  1886,  1887,  1888,  1891,  and  1892,  and  the  cholera  outbreaks 
of  1871  and  1893,  but  also  the  winter  outbreaks  of  cholera  infantum 
which  have  been  so  often  repeated.  It  cannot  be  doubted  that 
these  phenomena  bear  the  relation  to  each  other  of  cause  and 
effect.  It  is  thus  explained  why  in  the  warm  winters  no  such  out- 
breaks have  taken  place,  and  also  why  the  cholera  infantum  in 
winter  is  not  parallel  in  Hamburg  and  Altona. 

"  A  farther  support  of  this  idea  is  furnished  by  Berlin,  where  in 
the  same  way  frost  has  repeatedly  interfered  with  filtration.  In 
the  following  table  are  shown  the  deaths  from  diarrhoea  and  cholera 
infantum  for  a  few  winter  periods  having  unusual  increases  in  mor- 
tality in  comparison  with  the  bacteria  in  the  water-supply."  (These 
tables  show  that  in  March,  1886,  March,  1888,  February — March, 
1889,  and  February,  1891,  high  numbers  of  bacteria  resulted  from 
frost  disturbance  at  the  Stralau  works,  and  in  every  case  they  were 
followed  by  greatly  increased  death-rates  from  diarrhceal  diseases, 
—A.  H.) 

"  No  one  who  sees  this  exhibition  can  doubt  that  here  also  the 
supply  of  inadequately  purified  water  has  every  time  cost  the  lives 
of  many  children."  (100  to  400  or  more  each  time. — A.  H.)  "  Even 
more  conclusive  is  the  evidence,  published  by  the  Berlin  Health 
Office,  that  this  increase  was  confined  to  those  parts  of  the  city  sup- 
plied from  Stralau  "  (with  open  filters. — A.  H.),  "  and  that  the  parts 
supplied  from  the  better  Tegel  works  took  no  part  in  the  outbreaks, 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

which  was  exactly  the  case  with  the  well-known  typhoid  epidemic 
of  February  and  March,  1889.  ...  It  was  also  found  that  those 
children  nursed  by  their  mothers  or  by  wet-nurses  did  not  surfer, 
but  only  those  fed  on  the  milk  of  animals  or  other  substitutes,  and 
which  in  any  case  were  mixed  with  more  or  less  water." 

Under  Cholera,  page  28,  he  says :  "  The  revised  statistics  here 
given  differ  slightly  from  preliminary  figures  previously  issued  and 
widely  published."  (The  full  tables,  which  cannot  be  here  repro- 
duced, show  16,956  cases  and  8605  deaths.  8146  of  the  deaths 
occurred  in  the  month  ending  September  21.  Of  these,  1799 
were  under  5  years  old ;  776  were  5  to  15;  744,  15  to  25;  3520,  25  to 
50 ;  1369,  50  to  70  ;  and  397  over  70  or  of  unknown  age.  The  bulk 
of  the  cases  were  thus  among  mature  people,  children,  except  very 
young  children,  suffering  the  least  severely  of  any  age  class.) 

"  The  epidemic  began  on  August  16,  in  the  port  where  earlier 
outbreaks  have  also  had  their  origin.  The  original  source  of  the 
infection  has  not  been  ascertained  with  certainty,  but  was  probably 
from  one  of  two  sources.  Either  it  came  from  certain  Jews,  just 
arrived  from  cholera-stricken  Russia,  who  were  encamped  in  large 
numbers  near  the  American  pier,  or  the  infection  came  from  Havre, 
where  cholera  had  been  present  from  the  middle  of  July.  Perhaps 
the  germs  came  in  ships  in  water-ballast  which  was  discharged  at 
Hamburg,  which  is  so  much  more  probable,  as  the  sewage  of  Havre 
is  discharged  directly  into  the  docks. 

"  It  is  remarkable  that  in  Altona,  compared  to  the  total 
number  of  cases,  very  few  children  had  cholera,  while  in  the  epidemic 
of  1871  the  children  suffered  severely.  This  may  be  explained  by 
supposing  that  the  cholera  of  1892  in  Altona  was  not  introduced 
by  water,  but  by  other  means  of  infection.  .  .  . 

"  It  is  well  known  that  the  drinking-water  (of  Hamburg)  is  sup- 
posed to  have  been  from  the  first  the  carrier  of  the  cholera-germs. 
In  support  of  this  view  the  following  points  are  especially  to  be 
noted  : 

"  i.  The  explosive  rapidity  of  attack.     The  often-compared  epi- 


APPENDIX  II.  149 

demic  in  Munich  in  1854,  which  could  not  have  come  from  the 
water  is  characteristically  different  in  that  its  rise  was  much  slower 
and  was  followed  by  a  gradual  decline.  In  Hamburg,  with  six 
times  as  large  a  population,  the  height  of  the  epidemic  was  reached 
August  27,  only  12  days  after  the  first  cases  of  sickness,  while  in 
Munich  25  days  were  required.  In  Hamburg  also  the  bulk  of  the 
cases  were  confined  to  12  days,  from  August  25  to  September  5, 
while  in  Munich  the  time  was  twice  as  long. 

"  2.  The  exact  limit  of  the  epidemic  to  the  political  boundary 
between  Hamburg  and  Altona  and  Wandsbeck,  which  also  agrees 
with  the  boundary  between  the  respective  water-supplies,  while 
other  differences  were  entirely  absent.  Hamburg  had  for  1000  in- 
habitants 26.31  cases  and  13.39  deaths,  but  Altona  only  3.81  cases 
and  2.13  deaths,  and  Wandsbeck  3.06  cases  and  2.09  deaths.  .  . 

"  3.  The  old  experience  of  cholera  in  fresh-water  ports,  and  the 
analogy  of  many  earlier  epidemics.  In  this  connection  the  above- 
mentioned  epidemic  of  1871  in  Altona  has  a  special  interest,  even 
though  some  of  the  conclusions  of  Bockendahl's  in  his  report  of 
1871  are  open  to  objection.  First  there  were  3  deaths  August  3, 
which  were  not  at  once  followed  by  others.  Then  unfiltered  Elbe 
water  was  supplied  August  II  to  18.  On  the  I9th  an  outbreak  of 
cholera  extended  to  all  parts  of  the  city,  which  reached  its  height 
August  25  and  26,  and  afterwards  gradually  decreased.  In  all  105 
persons  died  of  cholera  and  186  (179  of  them  children)  of  diarrhoea. 
In  Hamburg,  four  times  as  large,  only  141  persons  died  of  cholera 
at  this  time,  thus  proportionately  a  smaller  number.  The  condi- 
tions were  then  the  reverse  of  those  of  1892,  an  infection  of  the 
Altona  water  and  a  comparative  immunity  in  Hamburg. 

"  It  is  objected  that  the  cholera-germs  were  not  found  in  the 
water  in  1892.  To  my  knowledge  they  were  first  looked  for,  and 
then  with  imperfect  methods,  in  the  second  half  of  September.  In 
the  after-epidemics  at  Altona,  they  were  found  in  the  river-water 
by  R.  Koch  by  the  use  of  better  methods. 

"  It  is  quite  evident  that  the  germs  were  also  distributed  by  other 


ISO  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

methods  than  by  the  city  water,  especially  by  dock-laborers  who 
became  infected  while  at  their  work  and  thus  set  up  little  secondary 
epidemics  where  they  went  or  lived.  .  .  .  These  laborers  and 
sailors,  especially  on  the  smaller  river-boats,  had  an  enormously 
greater  proportionate  amount  of  cholera  than  others.  .  .  .  These 
laborers  do  not  live  exclusively  near  the  water,  but  to  a  measure  in 
all  parts  of  the  city."  (And  in  Altona  and  Wandsbeck. — A.  H.) 

1 "  Altona  had  5  deaths  from  cholera  December  25  to  January  4, 
and  19  January  23  to  February  n,  and  no  more.  As  noted  above, 
this  is  attributed  to  the  water-supply,  and  to  defective  filtration  in 
presence  of  frost.  .  .  . 

"  The  cholera  could  never  have  reached  the  proportion  which  it 
did,  had  the  improvements  in  the  drinking-water  been  earlier  com- 
pleted." 

Further  accounts  of  the  water-supplies  of  Altona  and  of  Ham- 
burg and  of  the  new  filtration  works  at  the  latter  city  are  given  in 
Appendices  VII  and  VIII. 


APPENDIX  III.  151 


APPENDIX  III. 
METHODS   OF  SAND-ANALYSIS. 

(From  the  Annual  Report  of  the  Massachusetts  State  Board  of  Health  for  1892.) 

A  KNOWLEDGE  of  the  sizes  of  the  sand-grains  forms  the  basis  of 
many  of  the  computations.  This  information  is  obtained  by  means 
of  mechanical  analyses.  The  sand  sample  is  separated  into  por- 
tions having  grains  of  definite  sizes,  and  from  the  weight  of  the 
several  portions  the  relative  quantities  of  grains  of  any  size  can  be 
computed. 

Collection  of  Samples. — In  shipping  and  handling,  samples  of 
sand  are  best  kept  in  their  natural  moist  condition,  as  there  is  then 
no  tendency  to  separation  into  portions  of  unequal-sized  grains. 
Under  no  circumstances  should  different  materials  be  mixed  in  the 
same  sample.  If  the  material  under  examination  is  not  homogene- 
ous, samples  of  each  grade  should  be  taken  in  separate  bottles,  with 
proper  notes  in  regard  to  location,  quantity,  etc.  Eight-ounce 
wide-necked  bottles  are  most  convenient  for  sand  samples,  but  with 
gravels  a  larger  quantity  is  often  required.  Duplicate  samples  for 
comparison  after  obtaining  the  results  of  analyses  are  often  useful. 

Separation  into  Portions  having  Grains  of  Definite  Sizes. 
—Three  methods  are  employed  for  particles  of  different  sizes — 
hand-picking  for  the  stones,  sieves  for  the  sands,  and  water  elutria- 
tion  for  the  extremely  fine  particles.  Ignition,  or  determination  of 
albuminoid  ammonia,  might  be  added  for  determining  the  quantity 
of  organic  matter,  which,  as  a  matter  of  convenience,  is  assumed  to 
consist  of  particles  less. than  o.oi  millimeter  in  diameter. 


152  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

The  method  of  hand-picking  is  ordinarily  applied  only  to  parti- 
cles which  remain  on  a  sieve  two  meshes  to  an  inch.  The  stones  of 
this  size  are  spread  out  so  that  all  are  in  sight,  and  a  definite  num- 
ber of  the  largest  are  selected  and  weighed.  The  diameter  is  calcu- 
lated from  the  average  weight  by  the  method  to  be  described, 
while  the  percentage  is  reckoned  from  the  total  weight  Another 
set  of  the  largest  remaining  stones  is  then  picked  out  and  weighed 
as  before,  and  so  on  until  the  sample  is  exhausted.  With  a  little 
practice  the  eye  enables  one  to  pick  out  the  largest  stones  quite 
accurately. 

With  smaller  particles  this  process  becomes  too  laborious,  on 
account  of  the  large  number  of  particles,  and  sieves  are  therefore 
used  instead.  The  sand  for  sifting  must  be  entirely  free  from  mois 
ture,  and  is  ordinarily  dried  in  an  oven  at  a  temperature  somewhat 
above  the  boiling-point  The  quantity  taken  for  analysis  should 
rarely  exceed  100-200  grams.  The  sieves  are  made  from  carefully- 
selected  brass-wire  gauze,  having,  as  nearly  as  possible,  square  and 
even-sized  meshes.  The  frames  are  of  metal,  fitting  into  each  other 
so  that  several  sieves  can  be  used  at  once  without  loss  of  material. 
It  is  a  great  convenience  to  have  a  mechanical  shaker,  which  will 
take  a  series  of  sieves  and  give  them  a  uniform  and  sufficient  shak- 
ing in  a  short  time ;  but  without  this  good  results  can  be  obtained 
by  hand-shaking.  A  series  which  has  proved  very  satisfactory  has 
sieves  with  approximately  2,  4,  6,  10,  20,  40,  70,  100,  140,  and  200 
meshes  to  an  inch ;  but  the  exact  numbers  are  of  no  consequence, 
as  the  actual  sizes  of  the  particles  are  relied  upon,  and  not  the  num- 
ber of  meshes  to  an  inch. 

It  can  be  easily  shown  by  experiment  that  when  a  mixed  sand 
is  shaken  upon  a  sieve  the  smaller  particles  pass  first,  and  as  the 
shaking  is  continued  larger  and  larger  particles  pass,  until  the  limit 
is  reached  when  almost  nothing  will  pass.  The  last  and  largest  par- 
ticles passing  are  collected-  and  measured,  and  they  represent  the 
separation  of  that  sieve.  The  size  of  separation  of  a  sieve  bears 
tolerably  definite  relation  to  the  size  of  the  jnesh,  but  the  relatioi 


APPENDIX  III.  153 

is  not  to  be  depended  upon,  owing  to  the  irregularities  in  the 
meshes  and  also  to  the  fact  that  the  finer  sieves  are  woven  on  a  dif- 
ferent pattern  from  the  coarser  ones,  and  the  particles  passing  the 
finer  sieves  are  somewhat  larger  in  proportion  to  the  mesh  than  is 
the  case  with  the  coarser  sieves.  For  these  reasons  the  sizes  of  the 
sand-grains  are  determined  by  actual  measurements,  regardless  of 
the  size  of  the  mesh  of  the  sieve. 

It  has  not  been  found  practicable  to  extend  the  sieve-separations 
to  particles  below  o.io  millimeter  in  diameter  (corresponding  to  a 
sieve  with  about  200  meshes  to  an  inch),  and  for  such  particles 
elutriation  is  used.  The  portion  passing  the  finest  sieve  contains 
the  greater  part  of  the  organic  matter  of  the  sample,  with  the  ex- 
ception of  roots  and  other  large  undecomposed  matters,  and  it  is 
usually  best  to  remove  this  organic  matter  by  ignition  at  the  lowest 
possible  heat  before  proceeding  to  the  water-separations.  The  loss 
in  weight  is  regarded  as  organic  matter,  and  calculated  as  below 
o.oi  millimeter  in  diameter.  In  case  the  mineral  matter  is  decom- 
posed by  the  necessary  heat,  the  ignition  must  be  omitted,  and  an 
approximate  equivalent  can  be  obtained  by  multiplying  the  albu- 
minoid ammonia  of  the  sample  by  50.*  In  this  case  it  is  necessary 
to  deduct  an  equivalent  amount  from  the  other  fine  portions,  as 
otherwise  the  analyses  when  expressed  in  percentages  would  add 
up  to  more  than  one  hundred. 

Five  grams  of  the  ignited  fine  particles  are  put  in  a  beaker  90 
millimeters  high  and  holding  about  230  cubic  centimeters.  The 
beaker  is  then  nearly  filled  with  distilled  water  at  a  temperature  of 
20°  C,  and  thoroughly  mixed  by  blowing  into  it  air  through  a  glass 
tube.  A  larger  quantity  of  sand  than  5  grams  will  not  settle  uni- 
formly in  the  quantity  of  water  given,  but  less  can  be  used  if  de- 
sired. The  rapidity  of  settlement  depends  upon  the  temperature  of 
the  water,  so  that  it  is  quite  important  that  no  material  variation  in 
temperature  should  occur.  The  mixed  sand  and  water  is  allowed 

*  The  method  of  making  this  determination  was  given  in  the  American  Chemical 
Journal,  vol.  12,  p.  427. 


154  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

to  stand  for  fifteen  seconds,  when  most  of  the  supernatant  liquid, 
carrying  with  it  the  greater  part  of  the  particles  less  than  0.08  milli- 
meter, is  rapidly  decanted  into  a  suitable  vessel,  and  the  remaining 
sand  is  again  mixed  with  an  equal  amount  of  fresh  water,  which  is 
again  poured  off  after  fifteen  seconds,  carrying  with  it  most  of  the 
remaining  fine  particles.  This  process  is  once  more  repeated,  after 
which  the  remaining  sand  is  allowed  to  drain,  and  is  then  dried  and 
weighed,  and  calculated  as  above  0.08  millimeter  in  diameter.  The 
finer  decanted  sand  will  have  sufficiently  settled  in  a  few  minutes, 
and  the  coarser  parts  at  the  bottom  are  washed  back  into  the 
beaker  and  treated  with  water  exactly  as  before,  except  that  one 
minute  interval  is  now  allowed  for  settling.  The  sand  remaining  is 
calculated  as  above  0.04  millimeter,  and  the  portion  below  0.04  is 
estimated  by  difference,  as  its  direct  determination  is  very  tedious, 
and  no  more  accurate  than  the  estimation  by  difference  when  suffi- 
cient care  is  used. 

Determination  of  the  Sizes  of  the  Sand-grains.— The  sizes 
of  the  sand-grains  can  be  determined  in  either  of  two  ways — from 
the  weight  of  the  particles  or  from  micrometer  measurements* 
For  convenience  the  size  of  each  particle  is  considered  to  be  the 
diameter  of  a  sphere  of  equal  volume.  When  the  weight  and  spe- 
cific gravity  of  a  particle  are  known,  the  diameter  can  be  readily 
calculated.  The  volume  of  a  sphere  is  %7fds,  and  is  also  equal  to 
the  weight  divided  by  the  specific  gravity.  With  the  Lawrence 
materials  the  specific  gravity  is  uniformly  2.65  within  very  narrow 

limits,  and  we  have  -^-  =  \nd\  Solving  for  d  we  obtain  the  for- 
mula d  —  .t)&Vwy  where  d  is  the  diameter  of  a  particle  in  millime- 
ters and  w  its  weight  in  milligrams.  As  the  average  weight  of  par- 
ticles, when  not  too  small,  can  be  determined  with  precision,  this 
method  is  very  accurate,  and  altogether  the  most  satisfactory  for 
particles  above  o.io  millimeter;  that  is,  for  all  sieve  separations. 
For  the  finer  particles  the  method  is  inapplicable,  on  account  of 
the  vast  number  of  particles  to  be  counted  in  the  smallest  portion 


APPENDIX  III.  155 

which  can  be  accurately  weighed,  and  in  these  cases  the  sizes  are 
'determined  by  micrometer  measurements.  As  the  sand-grains  are 
not  spherical  or  even  regular  in  shape,  considerable  care  is  required 
to  ascertain  the  true  mean  diameter.  The  most  accurate  method  is 
to  measure  the  long  diameter  and  the  middle  diameter  at  right  an- 
gles to  it,  as  seen  by  a  microscope.  The  short  diameter  is  obtained 
by  a  micrometer  screw,  focussing  first  upon  the  glass  upon  which 
the  particle  rests  and  then  upon  the  highest  point  to  be  found.  The 
mean  diameter  is  then  the  cube  root  of  the  product  of  the  three 
observed  diameters.  The  middle  diameter  is  usually  about  equal 
to  the  mean  diameter,  and  can  generally  be  used  for  it,  avoiding  the 
troublesome  measurement  of  the  short  diameters. 

The  sizes  of  the  separations  of  the  sieves  are  always  determined 
from  the  very  last  sand  which  passes  through  in  the  course  of  an 
analysis,  and  the  results  so  obtained  are  quite  accurate.  With  the 
elutriations  average  samples  are  inspected,  and  estimates  made  of 
the  range  in  size  of  particles  in  each  portion.  Some  stray  particles 
both  above  and  below  the  normal  sizes  are  usually  present,  and 
even  with  the  greatest  care  the  result  is  only  an  approximation  to 
the  truth ;  still,  a  series  of  results  made  in  strictly  the  same  way 
should  be  thoroughly  satisfactory,  notwithstanding  possible  moder- 
ate errors  in  the  absolute  sizes. 

Calculation  of  Results. — When  a  material  has  been  separated 
into  portions,  each  of  which  is  accurately  weighed,  and  the  range  in 
the  sizes  of  grains  in  each  portion  determined,  the  weight  of  the 
particles  finer  than  each  size  of  separation  can  be  calculated,  and 
with  enough  properly  selected  separations  the  results  can  be 
plotted  in  the  form  of  a  diagram,  and  measurements  of  the  curve 
taken  for  intermediate  points  with  a  fair  degree  of  accuracy.  This 
curve  of  results  may  be  drawn  upon  a  uniform  scale,  using  the 
actual  figures  of  sizes  and  of  per  cents  by  weight,  or  the  logarithms 
of  the  figures  may  be  used  in  one  or  both  directions.  The  method 
of  plotting  is  not  of  vital  importance,  and  the  method  for  any  set  of 
materials  which  gives  the  most  easily  and  accurately  drawn  curves 


1 56 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


is  to  be  preferred.  In  the  diagram  published  in  the  Report  of 
the  Mass.  State  Board  of  Health  for  1891,  page  430,  the  logarith- 
mic scale  was  used  in  one  direction,  but  in  many  instances  the 
logarithmic  scale  can  be  used  to  advantage  in  both  directions. 
With  this  method  it  has  been  found  that  the  curve  is  often  almost 
a  straight  line  through  the  lower  and  most  important  section,  and 
very  accurate  results  are  obtained  even  with  a  smaller  number  of 
separations. 

Examples  of  Calculation  of  Results. — Following  are  exam- 
ples of  representative  analyses,  showing  the  method  of  calculation 
used  with  the  different  methods  of  separation  employed  with 
various  materials. 

I.    ANALYSIS   OF  A   GRAVEL  BY   HAND-PICKING,  11,8/0  GRAMS 
TAKEN   FOR   ANALYSIS. 


Number  of  Stones 
in  Portion. 
(Largest  Selected 
Stones.) 

Total 
Weight  of 
Portion. 
Grams. 

Average 
Weight  of 
Stones. 
Milligrams. 

Estimated 
Weight  of 
Smallest 
Stones. 
Milligrams. 

Corre- 
sponding 
Size. 
Milli- 
meters. 

Total 
Weight  of 
Stones 
Smaller  than 
this  Size. 

Per  Cent  of 
Total 
Weight 
Smaller  than 
this  Size. 

1  1  870 

100 

IO  

^,^2O 

332,000 

250,000 

56 

8  sso 

72 

10  

I.Q3O 

IQ3,OOO 

165,000 

49 

6  620 

56 

IO  

I  -*8O 

138  ooo 

124  ooo 

45 

c  2A.O 

44 

2O  

2,2OO 

I  lO.OOO 

Q-3  OOO 

41 

$>•<•'¥-' 
'i  OAO 

26 

2O  

I.52O 

76,000 

64  ooo 

36 

I   52O 

13 

2O  

I,OOO 

5O,OOO 

36,000 

30 

5  2O 

4.4 

2O  

4.6O 

23  ooo 

IO  OOO 

20 

J~. 
60 

.5 

IO            ,          ... 

4-O 

4.  ooo 

2  OOO 

11 

2O 

,2 

Dust  

2O 

The  weight  of  the  smallest  stones  in  a  portion  given  in  the 
fourth  column  is  estimated  in  general  as  about  half-way  between 
the  average  weight  of  all  the  stones  in  that  portion  and  the  average 
weight  of  the  stones  in  the  next  finer  portion. 

The  final  results  are  shown  by  the  figures  in  full-faced  type  in 
the  last  and  third  from  the  last  columns.  By  plotting  these  figures 
we  find  that  10  per  cent  of  the  stones  are  less  than  35  millimeters 
in  diameter,  and  60  per  cent  are  less  than  51  millimeters.  The 
"  uniformity  coefficient,"  as  described  below,  is  the  ratios  of  these 
numbers,  or  1.46,  while  the  "effective  size"  is  35  millimeters. 


APPENDIX  III. 


157 


II.  ANALYSIS  OF  A  SAND  BY  MEANS  OF  SIEVES. 
A  portion  of  the  sample  was  dried  in  a  porcelain  dish  in  an  air- 
bath.  Weight  dry,  110.9  grams.  It  was  put  into  a  series  of  sieves 
in  a  mechanical  shaker,  and  given  one  hundred  turns  (equal  to 
about  seven  hundred  single  shakes).  The  sieves  were  then  taken 
apart,  and  the  portion  passing  the  finest  sieve  weighed.  After 
noting  the  weight,  the  sand  remaining  on  the  finest  sieve,  but  pass- 
ing all  the  coarser  sieves,  was  added  to  the  first  and  again  weighed, 
this  process  being  repeated  until  all  the  sample  was  upon  the 
scale,  weighing  1 10.7  grams,  showing  a  loss  by  handling  of  only  0.2 
gram.  The  figures  were  as  follows : 


Size  of 

Size  of 

Sieve 

Separation 
of  this 

Quantity 
of  Sand 

Per  Cent 
of 

Sieve 

Separation 
of  this 

Quantity 
of  Sand 

Per  Cent 
of 

Marked. 

Sieve. 
Milli- 

Passing. 
Grams. 

Total 
Weight. 

Marked. 

Sieve. 
Milli- 

Passing. 
Grams. 

Total 
Weight. 

meters. 

meters. 

TOO 

,105 

c 

.5 

4O.  . 

.46 

56  7 

51.2 

I4.O 

.135 

I    -\ 

1.2 

2O  

.93 

89.1 

80.5 

IOO 

.182 

41 

3.7 

IO            ... 

2.04 

104.  6 

94.3 

60 

.320 

2T.    2 

21.0 

6  

3.90 

I  IO  7 

100.0 

Plotting  the  figures  in  heavy-faced  type,  we  find  from  the  curve 
that  10  and  60  per  cent  respectively  are  finer  than  .25  and  .62  milli- 
meter, and  we  have  for  effective  size,  as  described  above,  .25,  and 
for  uniformity  coefficient  2.5. 

III.    ANALYSIS   OF  A  FINE   MATERIAL  WITH   ELUTRIATION. 

The  entire  sample,  74  grams,  was  taken  for  analysis.  The  sieves 
used  were  not  the  same  as  those  in  the  previous  analysis,  and 
instead  of  mixing  the  various  portions  on  the  scale  they  were 
separately  weighed.  The  siftings  were  as  follows  : 

Remaining  on  sieve  marked    10,  above  2.2    millimeters 1.5  grams 

20,      "        .98  "         7-o 

40,      "        .46  "         22.0 

«  "          «  "  70,         "  .24  "  20.2         " 

«          "  "         140,         "  .13  "  ....       9-2          " 

Passing  sieve  140,   below  .13  "         14-1 


I58 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


The  14.1  grams  passing  the  140  sieve  were  thoroughly  mixed, 
and  one  third,  4.7  grams,  taken  for  analysis.  After  ignition  just 
below  a  red  heat  in  a  radiator,  the  weight  was  diminished  by  0.47 
gram.  The  portion  above  .08  millimeter  and  between  .04  and  .08 
millimeter,  separated  as  described  above,  weighed  respectively  1.27 
and  1,71  grams,  and  the  portion  below  .04  millimeter  was  esti- 
mated by  difference  [4.7  —  (0.47  +  1.27  -f-  1.71)]  to  be  1.25  grams. 
Multiplying  these  quantities  by  3,  we  obtain  the  corresponding 
quantities  for  the  entire  sample,  and  the  calculation  of  quantities 
finer  than  the  various  sizes  can  be  made  as  follows : 


Size  of  Grain. 

Weight. 
Grams. 

Size  of 
Largest 
Particles. 
Millimeters. 

Weight  ol  all 
the  Finer 
Particles. 
Grams. 

Per  Cent  by 
Weight  of 
all  Finer 
Particles. 

Above  2.  20  millimeters  

I  .  SO 

74.OO 

100 

.98-2.20 

7  oo 

2.20 

72    CO 

98 

.46-  .98                    

22   OO 

.98 

/*«  5^ 

6c.  Co 

89 

.24-  .46 

2O.2O 

.46 

43.  co 

60 

.  I  "V-     24 

Q    2O 

.24 

23    3O 

32 

.08-  .13 

y  .  **? 

3  81 

.13 

14    IO 

19 

.04-   .08 

c  13 

.08 

IO   2O 

14 

,OI-   .04                          

3    7C 

.04 

C.l6 

7 

Loss  on  ignition  (assumed  to  be 
less  than  .01  millimeter)  

I  .41 

.01 

I  .41 

1.9 

By  plotting  the  heavy-faced  figures  we  find  that  IO  and  60  per 
cent  are  respectively  finer  than  .055  and  .46  millimeter,  and  we  have 
effective  size  .055  millimeter  and  uniformity  coefficient  8. 

The  effective  size  and  uniformity  coefficient  calculated  in  this 
way  have  proved  to  be  most  useful  in  various  calculations,  particu- 
larly in  estimating  the  friction  between  the  sands  and  gravels  and 
water.  The  remainder  of  the  article  in  the  Report  of  the  Mass. 
State  Board  of  Health  is  devoted  to  a  discussion  of  these  relations 
which  were  mentioned  in  Chapter  III  of  this  volume. 


APPENDIX  IV. 


159 


APPENDIX   IV. 

A  LIST  OF  SOME   FILTRATION   WORKS. 

(The  numbers  in  parenthesis  indicate  the  years  to  which  the  statistics  refer.   (2)  =  1892,  (4)  =  1894,  etc.) 


Place. 

Population 
Supplied. 

Filters. 

Nominal 
Capacity 
of  Works. 

Average  Daily 
Consumption 
for  One  Year. 

No. 

Aver- 
age 
Area. 
Acres. 

Total 
Area. 
Acres. 

Altona 

I$6,000 
|  515,000(4)  | 
200,000 

<    1,  606  ,000  > 

364,000 
146,000 
335.000 
105,000 
500,000 
(100,000) 

12 

4 

8 
22 
II 
21 
6 

0.21 
!-34 

0.68 
0.26 

0.57 
0.84 
0.59 

0.77 

2.52  (4) 

5-35  (4) 
4.80(4) 
2.10  (4) 
12.70  (4) 

9-15  (4) 

12.40(4) 
4.62  (4) 

2-35(2) 

5-12(4) 
1.46(2) 

3-00(4) 
2.31  (4) 
2.85(4) 
1.32  (4) 
0.56  (4) 
5.00(4) 

2.00 

2.88(4) 
34-00(4) 

3-13  (4) 
0.96(4) 

4-95  (4) 

4,200,000  (2) 

Amsterdam  :  Vecht 
Dune. 
Antwerp  

10,600,000 

6,300,000  (3) 
1,720,000(3) 

Berlin  :  Mtiggel  

23,000,000 

Stralau 

9,700,000  (3) 
19,400,000  (3) 
12,000,000 
2,850,000  (2) 
7,300,000  (2) 
2,070,000  (2) 
20,400,000  (2) 

Tegel 

23,000,000 

Bradford  

Bremen  

5 

I  .02 

Brunswick  

Budapest  

8 
15 
9 

7 

2 
10 

4 
6 
18 
8 
6 
3 

o.37 
0.15 
0.32 
0.19 
0.28 
0.50 
0.50 
0.48 
1.89 

0.39 
o.  16 

1.65 

Choisy  le  Roi  

7,900,000 

Copenhagen  

Darlington  

40,000 
34.ooo 
340,000 
794,000 
170,000 
583,000 
167,000 
48,000 
815,000 

2,000,000  (0) 

880,000  (3) 
17,900,000 

Dordrecht  

Dublin.         .      . 

Edinburgh    

Hague  

4,200,000  (3) 

31,600,000  (3) 

2,570,000  (2) 

1,340,000(3) 

Hamburg  

Konigsberg  

Liegnitz  

Liverpool  :  Oswestry 
Rivington 
London,  all  filters.  .. 
Chelsea 

•     5,030,000 

64,000 
200,000 
180,000 
240,000 

"3 
7 
3i 
15 

10 
20 

18 

12 

7 
ii 
ii 
18 

1.02 
0.96 
0.94 

1.18 

o.95 
0.82 

1.14 
1.25 
0.36 
0.36 
0.30 
0-35 

115-75(4) 
6.75(4) 
29-75  (4) 
17-75(4) 
9  -5°  (4) 
16.50  (4) 
20.50(4) 
15.00(4) 
2  .  54  (4) 
3.90(4) 
3-27(4) 
6.30(4) 

i95,'ooo,ooo*(3) 
13,400,000  (3) 
52,700,000  (3) 
21,800,000  (3) 
25,400,000  (3) 
44,800,000  (3) 
36,600,000  (3) 
22,400,000  (3) 
3,400,000  (i) 
5,100,000  (2) 

1  1  ,000,000  (O) 

13,300,000(3) 

E.  London 

Grand  Junction.  . 
Lambeth  

New  River 

Southw.  &  Vauxh. 
W.  Middlesex.... 
Liibeck  

Magdeburg  

Middlesborough  
Rotterdam 

*  This  is  filtered  water  only;  some  of  the  companies  secure  some  ground-water, 
which  they  mix  with  the  filtered  water,  and  which  is  included  in  the  quantities  for  the 
separate  companies,  and  as  I  have  no  records  of  these  amounts  to  make  the  correc- 
tions, the  latter  quantities  are  slightly  too  great. 


i6o 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 


SOME   FILTRATION   WORKS — Continued. 


Place. 

Population 
Supplied. 

Filters. 

Nominal 
Capacity 
of  Works. 

Average  Daily 
Consumption 
for  One  Year. 

No. 

Aver- 
age 
Area. 
Acres. 

Total 
Area. 
Acres. 

Schiedam  .         .... 

25,000 
139,000 
960,000 
500,000 
70,000  + 
100,000 

5 

7 
ii 

12 

6 

7 

O.26 
0.15 

o.53 
0.52 

o.34 
0.17 

1.33(4) 
1.05(2) 
5.85(0) 

6  .  20  (o) 

2.04(4) 

1.18(4) 

680,000  (3) 
2,5OO,OOO  (2) 

39,000,000  (o) 
6,100,000  (o) 

Stuttgart  

St.  Petersburg.  .   .  . 

Warsaw  

York 

Zurich  .... 

5,500,000(2) 

STATISTICS   OF  OPERATION  OF   SOME  FILTERS   FOR   ONE  YEAR. 


Place. 

Year.* 

Total  Quantity. 
Millions  of 
Gallons. 

Area  of  Filters. 
Acres. 

$ 

5*3.- 

|s£33 

Period  between 
Scraping  Days. 

Area  of  Filter- 
surface 
Cleaned.  Acres. 

Million  Gallons 
per  Acre  Fil- 
tered between 
Scrapings. 

i 
j 

« 
8 

1 

in 

I 

Berlin,  all  filters.. 
Tegel. 

1892 
1893 
1893 
1892 
1892 
1892 
1892 
1892 
1892 
1891 
1892 
1892 
1891 
1892 
1891 
1891 
1892 
1892 
1892 
1892 
1892 
1892 
1892 
1892 

9,600 
7,070 
3,510 
2,660 
7,360 

1,835 
1,040 
1,520 
1,000 

752 
907 

1,360 

1,995 
340 
1,225 
254 
65,783t 
4,164 
17,782 
8,165 
8,727 
15,224 
11,510 
7,660 

21.55 
12.40 

9.15 
4.12 

3.00 

2.48 

S2.35 
2.18 
1.98 
1.46 
1.05 

•  95 
.84 
•  70 
.52 
•  37 
109.75 

6.75 
29.75 

17.75 
9.50 
16.50 

14.5° 
15.00 

1.23 

1.57 
1.05 
1.76 
6.70 

2.  02 
I  .22 
1.92 

1-39 
1.41 
2.38 
3.90 
6.50 

1-34 
6.50 
1.88 
i  .64 

i.69 

i  .64 
1.26 
2.52 
2.52 
2.17 
i  .40 

i- 

74.2 
10.33 

55 
35 
50 
47 
73 
46 
17 
75 
37 

6 

19.0 
0.79 
8 

7 

10 

7 

20 
10 

4 
17 
8 

Stralau... 
Breslau  

37-3 
3-3 
25.1 

23.4 
26.3 

20.0 
40 
28 

9-5 
32 
15-4 

38.1 
254.0 

32.3 

37.o 

30-3 
28.8 

12.8 
12.8 

31.3 
7.7 

8.9 
19.0 
5-9 

70 
29 
57 
28 

51 

35 
59 
7i 
44 
260 

38 
65 

43 

157 
76 

9 
H 

7i 

Budapest   

Masfdeburer 

Altona  

Konigsberg  

Brunswick  .  .  . 

Stuttgart  

Stettin  

Zurich  

Posen    .... 

Liibeck  

Frankfort  . 

87 

10 

23.5 

London,  all  filters. 
Chelsea 

E  London 

Grand  Junction. 
Lambeth 

New  River  
South.  &  Vaux. 
W.  Middlesex.. 

*  These  results  are  for  12  months  in  every  case,  but  not  always  for  the  calendar 
year.     Many  German  results  are  for  the  year  ending  March  31. 
f  Filtered  water  only;  see  foot  note  on  page  159. 


APPENDIX    F.  l6l 


APPENDIX  V. 
LONDON'S  WATER-SUPPLY. 

LONDON  alone  among  great  capitals  is  supplied  with  water  by 
private  companies.  They  are,  however,  under  government  super- 
vision, and  the  rates  charged  for  water  are  regulated  by  law.  There 
are  eight  companies,  each  of  which  supplies  its  own  separate  dis- 
trict, so  that  there  is  no  competition  whatever.  One  of  the  compa- 
nies supplying  460,000  people  uses  only  ground-water  drawn  from 
deep  wells  in  the  chalk,  but  the  other  seven  companies  depend 
mainly  upon  the  rivers  Thames  and  Lea  for  their  water.  All 
water  so  drawn  is  filtered,  and  must  be  satisfactory  to  the  water 
examiner,  who  is  required  to  inspect  the  water  supplied  by  each 
company  at  frequent  intervals,  and  the  results  of  the  examinations 
are  published  each  month. 

In  1893  the  average  daily  supply  was  235,000,000  gallons,  of 
which  about  40,000,000  were  drawn  from  the  chalk,  125,000,000 
from  the  Thames,  and  70,000,000  from  the  Lea.  Formerly  some  of 
the  water  companies  drew  water  from  the  Thames  within  the  city 
where  it  was  grossly  polluted,  and  the  plagues  and  cholera  which 
formerly  ravaged  London  were  in  part  due  to  this  fact.  These 
intakes  were  abandoned  many  years  ago,  and  all  the  companies 
now  draw  their  water  from  points  outside  of  the  city  and  its  imme- 
diate suburbs. 

The  area  of  the  watershed  of  the  Thames  above  the  intakes  of 
the  water  companies  is  3548  square  miles,  and  the  population  living 
upon  it  in  1891  was  1,056,415.  The  Thames  Conservancy  Board 
has  control  of  the  main  river  for  its  whole  length,  and  of  all  tributa- 
ries within  ten  miles  in  a  straight  line  of  the  main  river,  but  has  no 


1  62  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

jurisdiction  over  the  more  remote  feeders.  The  area  drained  is 
•essentially  agricultural,  with  but  little  manufacturing,  and  there  are 
but  few  large  towns.  In  the  area  coming  under  the  conservators 
there  are  but  six  towns  with  populations  above  10,000  and  an  aggre- 
gate population  of  170,000,  and  there  are  but  two  or  three  other 
large  towns  on  the  remaining  area  more  than  ten  miles  from  the 
river.  These  principal  towns  are  as  follows  : 


PoputaUon  ,8,,. 

Reading  .  .................  .  60,054  49  miles 

Oxford  ....................  45,791  87  " 

New  Swindon  ..............  27,295  116  " 

High  Wycomb  .............  I3»435  33  " 

Windsor  ...................  12,327  18  " 

Maidenhead  ...............  10,607  25  " 

Guildford  ...................  14,319  20     " 

Guildford  is  outside  of  the  conservators'  area.  All  of  the  above 
towns  treat  their  sewage  by  irrigation. 

Among  the  places  that  are  regarded  as  the  most  dangerous  are 
Chertsey  and  Staines,  with  populations  of  9215  and  5060,  only  8  and 
ii  miles  above  the  intakes  respectively.  These  towns  are  only 
partially  sewered  and  still  depend  mainly  on  cesspools.  An  at- 
tempt is  made  to  treat  the  little  sewage  which  they  produce  upon 
land,  but  the  work  has  not  as  yet  been  systematically  carried  out. 
There  are  also  several  small  towns  of  3000  inhabitants  or  less  upon 
the  upper  river  which  do  not  treat  their  sewage  so  far  as  they 
have  any,  but,  owing  to  their  great  distance,  the  danger  from  them 
is  much  less  than  from  Chertsey  and  Staines.  Twenty-one  of  the 
principal  towns  upon  the  watershed  have  sewage  farms,  and  there 
are  no  chemical  precipitation  plants  now  in  use. 

Boats  upon  the  river  are  not  allowed  to  drain  into  it,  but  are 
compelled  to  provide  receptacles  for  their  sewage,  and  facilities  are 
provided  for  removing  and  disposing  of  it  ;  and  as  an  additional  pre- 
caution no  boat  is  allowed  to  anchor  within  five  miles  of  the  in- 
takes. 


APPENDIX    V.  163 

The  conservators  of  the  river  Lea  have  control  of  its  entire 
drainage  area,  which  is  about  460  square  miles,  measured  from  the 
East  London  water  intakes,  and  has  a  population  of  189,287.  On 
this  watershed  there  is  but  a  single  town  with  more  than  10,000  in- 
habitants, this  being  Lutton  near  the  headwaters  of  the  river,  with  a 
population  of  30,005.  The  sewage  from  Lutton  and  from  seven- 
teen smaller  places  is  treated  upcfn  land.  No  crude  sewage  is 
known  to  be  ordinarily  discharged  into  the  river.  At  Hereford, 
eleven  miles  above  the  East  London  intakes,  there  is  a  chemical 
precipitation  plant.  The  conservators  do  not  regard  this  treatment 
as  satisfactory,  and  have  recently  conducted  an  expensive  lawsuit 
against  the  local  authorities  to  compel  them  to  further  treat  their 
effluent.  The  suit  was  lost,  the  court  holding  that  no  actual  injury 
to  health  had  been  shown.  It  is  especially  interesting  to  note  that 
of  the  thirty-nine  places  on  the  Thames  and  the  Lea  giving  their 
sewage  systematic  treatment  there  is  but  a  single  place  using  chem- 
ical precipitation,  and  there  it  is  not  considered  satisfactory.  Form- 
erly quite  a  number  of  these  towns  used  other  processes  than  land 
treatment,  but  in  every  case  but  Hereford  land  treatment  has  been 
substituted. 

In  regard  to  the  efficiency  of  the  sewage  farms,  it  is  believed 
that  in  ordinary  weather  the  whole  of  the  sewage  percolates 
through  the  land,  and  the  inspectors  of  the  Conservancy  Boards 
strongly  object  to  its  being  allowed  to  pass  over  the  surface  into 
the  streams.  The  land,  however,  is  for  the  most  part  impervious, 
as  compared  to  Massachusetts  and  German  sewage  farms,  and  in 
times  of  heavy  storms  the  land  often  has  all  the  water  it  can  take 
without  receiving  even  the  ordinary  flow  of  sewage,  and  much  less 
the  increased  storm-flow.  At  such  times  the  sewage  either  does  go 
over  the  surface,  or  perhaps  more  frequently  is  discharged  directly 
into  the  rivers  without  even  a  pretence  of  treatment.  The  con- 
servators apparently  regard  this  as  an  unavoidable  evil  and  do  not 
vigorously  oppose  it.  It  is  the  theory  that,  owing  to  the  increased 
dilution  with  the  storm-flows,  the  matter  is  comparatively  harmless, 


164  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

although  it  would  seem  that  the  reduced  time  required  for  it  to 
reach  the  water-works  intakes  might  largely  offset  the  effect  of 
increased  dilution. 

The  water  companies  have  large  storage  and  sedimentation 
basins  with  an  aggregate  capacity  equal  to  nine  days'  supply,  but 
the  proportion  varies  wfdely  with  the  different  companies.  It  is 
desired  that  the  water  held  in  reserve  shall  be  alone  used  while  the 
river  is  in  flood,  as;  owing  to  its  increased  pollution,  it  is  regarded 
as  far  more  dangerous  than  the  water  at  other  times;  but  as  no 
record  is  kept  of  the  times  when  raw  sewage  is  discharged,  and  no 
exact  information  is  available  in  regard  to  the  times  when  the  com- 
panies do  not  take  in  raw  water,  it  can  safely  be  assumed  that  a 
considerable  amount  of  raw  sewage  does  become  mixed  with  the 
water  which  is  drawn  by  the  companies. 

The  water  drawn  from  the  river  is  filtered  through  113  filters 
having  an  area  of  116  acres.  None  of  the  filters  are  covered,  and 
with  an  average  January  temperature  of  39°  but  little  trouble  with 
ice  is  experienced.  A  few  new  filters  are  provided  with  appliances 
for  regulating  the  rate  on  each  filter  separately  and  securing  regular 
and  determined  rates  of  filtration,  but  nearly  all  of  the  filters  are  of 
the  simple  type  described  on  page  48,  and  the  rates  of  filtration 
are  subject  to  more  or  less  violent  fluctuation,  the  extent  of  which 
cannot  be  determined. 

The  area  of  filters  is  being  continually  increased  to  meet  increas- 
ing consumption  ;  the  approximate  areas  of  filters  in  use  having 
been  as  follows : 

1839 First  filters  built 

1855 37  acres 

1866 47     " 

1876 77     " 

1886 104    " 

1894 116    " 

There  has  been  a  tendency  to  reduce  somewhat  the  rate  of  filtra 
tion.  In  1868,  with  51  acres  of  filters,  the  average  daily  quantity  of 


APPENDIX    V.  165 

water  filtered  was  111,000,000  gallons,  or  2,180,000  gallons  per  acre. 
In  1884,  with  97  acres  of  filter  surface,  the  daily  quantity  filtered  was 
157,000,000  gallons,  or  1,620,000  gallons  per  acre  ;  and  in  1893,  with 
1 16  acres  of  filter  surface  and  195,000,000  gallons  daily,  the  yield 
per  acre  was  1,680,000  gallons. 

Owing  to  the  area  of  filter  surface  out  of  use  while  being  cleaned, 
the  variations  in  consumption  of  water,  and  the  imperfections  of  the 
regulating  apparatus,  the  actual  rates  of  filtration  are  often  very 
much  higher  and  at  times  may  easily  be  double  the  figures  given. 

Evidence  regarding  the  healthfulness  of  the  filtered  river-water 
was  collected  and  examined  in  a  most  exhaustive  manner  in  1893 
by  a  Royal  Commission  appointed  to  consider  the  water-supply  of 
the  metropolis  in  all  its  aspects  with  reference  to  future  needs.  This 
commission  was  unable  to  obtain  any  evidence  whatever  that  the 
water  as  then  supplied  was  unhealthy  or  likely  to  become  so,  and 
they  report  that  the  rivers  can  safely  be  depended  upon  for  many 
years  to  come. 

The  numbers  of  deaths  from  all  causes  and  from  typhoid  fever 
annually  per  million  of  inhabitants  for  the  years  1885-1891  in  the 
populations  receiving  their  waters  from  different  sources  in  London 
were  as  follows : 

w  ,  Deaths  from          Deaths  from 

All  Causes.        Typhoid  Fever. 

Filtered  Thames  water  only 19,501  125 

"  Lea  water  only 21,334  167 

Kent  wells  only 18,001  123 

Thames  and  Lea  jointly 18,945  138 

"     Kentjointly 18,577  133 

The  population  supplied  exclusively  from  the  Lea  by  the  East 
London  Company  is  of  a  poorer  class  than  that  of  the  rest  of  Lon- 
don, and  this  may  account  for  the  slightly  higher  death-rate  in  this 
section.  Aside  from  this  the  rate  is  remarkably  uniform  and  shows 
no  great  difference  between  the  section  drinking  ground-water  only 
and  those  drinking  filtered  river-xvaters.  The  death-rate  from 


1 66  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

* 

typhoid  fever  is  also  very  uniform  and,  although  higher  than  that  of 
some  Continental  cities  with  excellent  water-supplies  (Berlin,  Vienna, 
Munich,  Dresden),  is  very  low — lower  than  in  any  American  city  of 
which  I  have  records. 

In  this  connection,  it  was  shown  by  the  Registrar-General  that 
there  is  only  a  very  small  amount  of  typhoid  fever  on  the  water- 
sheds of  the  Thames  and  Lea,  so  that  the  danger  of  infection  of  the 
water  as  distinct  from  pollution  is  less  than  would  otherwise  be  the 
case.  Thus  for  the  seven  years  above  mentioned  the  numbers  of 
deaths  from  typhoid  fever  per  million  of  population  were  only  105 
and  1 20  on  the  watersheds  of  the  Thames  and  the  Lea  respectively, 
as  against  176  for  the  whole  of  England  and  Wales. 


APPENDIX    VI.  167 


APPENDIX    VI. 
THE  BERLIN  WATER-WORKS. 

THE  original  works  were  built  by  an  English  company  in  1856, 
and  were  sold  to  the  city  in  1873  for  $7,200,000. 

The  water  was  taken  from  the  river  Spree  at  the  Stralau  Gate, 
which  was  then  above,  but  is  now  surrounded  by,  the  growing  city. 
The  water  was  always  filtered,  and  the  original  filters  remained  in  use 
until  1893,  when  they  were  supplanted  by  the  new  works  at  Lake 
Miiggel.  Soon  after  acquiring  the  works  the  city  introduced  water 
from  wells  by  Lake  Tegel  as  a  supplementary  supply,  but  much 
trouble  was  experienced  from  crenothrix,  an  organism  growing  in 
ground-waters  containing  iron,  and  in  1883  tm*s  supply  was  replaced 
by  filtered  water  from  Lake  Tegel.  With  rapidly-increasing  pollu- 
tion of  the  Spree  at  Stralau  the  purity  of  this  source  was  questioned, 
and  in  1893  it  was  abandoned  (although  still  held  as  a  reserve  in  case 
of  urgent  necessity),  the  supply  now  being  taken  from  the  river  ten 
miles  higher  up,  at  Miiggel. 

The  watershed  of  the  Spree  above  Stralau,  as  I  found  by  map 
measurement,  is  about  3800  square  miles ;  the  average  rainfall  is 
about  25  inches  yearly.  At  extreme  low  water  the  river  discharges 
457  cubic  feet  per  second,  or  295  million  gallons  daily,  and  when  in 
flood  5700  cubic  feet  per  second  may  be  discharged.  The  city  is 
allowed  by  law  to  take  46  million  gallons  daily  for  water-supply,  and 
this  quantity  can  be  drawn  either  at  Stralau  or  at  Miiggel. 

Above  Stralau  the  river  is  polluted  by  numerous  manufactories 
and  washing  establishments,  and  by  the  effluent  from  a  considerable 
part  of  the  city's  extensive  sewage  farms.  The  shipping  on  this 
part  of  the  river  also  is  heavy,  and  sewage  from  the  boats  is  dis- 


1 68  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

charged  directly  into  the  river.  The  average  number  of  bacteria  in, 
the  Spree  at  this  point  is  something  over  ten  thousand  per  cubic 
centimeter,  and  99.6  per  cent  of  them  were  removed  by  the  filters 
in  1893. 

The  watershed  of  the  Spree  above  the  new  water-works  at 
Muggel  I  found  by  map  measurement  to  be  2800  square  miles,  and 
the  low  water-discharge  is  said  to  be  269  million  gallons  daily.  The 
river  at  this  point  flows  through  Lake  Miiggel,  which  forms  a  natural 
sedimentation-basin,  and  the  raw  water  is  quite  clear  except  in  windy 
weather. 

There  were  16  towns  on  the  watershed  with  populations  above 
2000  each  in  1890,  and  an  aggregate  population  of  132,000,  which 
does  not  include  the  population  of  the  smaller  places  or  country 
districts.  None  of  these  places  purify  their  sewage  so  far  as 
they  have  any.  Furstenwalde  with  a  population  of  12,935,  and 
22  miles  above  Muggel,  has  surface  sewers  discharging  directly  into 
the  river.  Above  Furstenwalde  the  river  runs  through  numerous 
lakes  which  probably  remove  the  effect  of  the  pollution  from  the  more 
distant  cities.  There  is  considerable  shipping  on  the  river  for  some 
miles  above  Furstenwalde  (which  forms  a  section  of  the  Friedrich 
Wilhelm  Canal),  but  hardly  any  between  Muggel  and  Furstenwalde. 
The  raw  water  at  Muggel  contains  two  or  three  hundred  bacteria 
per  cubic  centimeter,  and  is  thus  a  comparatively  pure  water  before 
filtration.  It  is  slightly  peaty  and  the  filtered  water  has  a  light  straw 
color. 

Lake  Tegel,  which  supplies  the  other  part  of  the  city's  supply,  is 
an  enlargement  of  the  river  Havel.  The  watershed  above  Tegel 
I  find  to  be  about  1350  square  miles,  and  the  annual  rainfall  is 
about  22  inches.  The  low  water-discharge  is  said  to  be  182  million 
gallons  daily,  and  the  city  is  allowed  by  law  to  take  23  million  gal- 
lons for  water-supply. 

There  were  ten  towns  upon  the  watershed  with  populations 
above  2000  each  in  1890,  and  with  an  aggregate  population  of  44,000. 
Of  these  Tegel  is  directly  upon  the  lake  with  a  population  of  3< 


APPENDIX    VI.  169 

and  Oranienburg,  14  miles  above,  has  a  population  of  6000  and  is 
rapidly  increasing.  The  shipping  on  the  lake  and  river  is  heavy. 
The  lake  water  ordinarily  contains  two  or  three  hundred  bacteria 
per  cubic  centimeter.  The  lake  is  shallow  and  becomes  turbid  in 
windy  weather. 

There  are  21  filter-beds  at  Tegel  with  a  combined  area  of  12.40 
acres  to  furnish  a  maximum  of  23  million  gallons  of  water  daily, 
and  22  filters  at  Mu'ggel  with  a  combined  area  of  12.7  acres  to  de- 
liver the  same  quantity.  Twenty-two  more  filters  will  be  built  at 
Miiggel  within  a  few  years  to  purify  the  full  quantity  which  can  be 
taken  from  the  river.  All  of  these  filters  are  covered  with  brick 
arches  supported  by  pillars  about  16  feet  apart  from  centre  to  centre 
in  each  direction,  and  the  whole  is  covered  by  nearly  3  feet  of  earth, 
making  them  quite  frost-proof.  The  original  filters  at  Stralau  were 
open,  but  much  difficulty  was  experienced  with  them  in  winter. 

The  bottom  of  the  filters  at  Tegel  consists  of  8  inches  of  concrete 
above  20  inches  of  packed  clay  and  with  2  inches  of  cement  above, 
and  slopes  slightly  from  each  side  to  the  centre.  The  central  drain 
goes  the  whole  length  of  the  filters  and  has  a  uniform  cross-section 
of  about  T^Vo  °f  tne  area  °f  tne  whole  bed.  There  are  no  lateral 
drains,  but  the  water  is  brought  to  the  central  drain  by  a  twelve- 
inch  layer  of  stones  as  large  as  a  man's  fist ;  above  this  there  is  an- 
other foot  of  gravel  of  graded  sizes  supporting  two  feet  of  fine  sand, 
which  is  reduced  by  scraping  to  half  its  thickness  before  the  sand  is 
replaced.  The  average  depth  of  water  above  the  sand  is  nearly  5  feet. 
The  filters  are  not  allowed  to  filter  at  a  rate  above  2.57  million  gal- 
lons per  acre  daily,  and  at  this  rate  with  70  per  cent  of  the  area  in 
service  the  whole  legal  quantity  of  water  can  be  filtered.  The  filters 
work  at  precisely  the  same  rate  day  and  night,  and  the  filtered  water 
is  continuously  pumped  as  filtered  to  ample  storage  reservoirs  at 
Charlottenburg.  The  pumps  which  lift  the  water  from  the  lake  to 
the  filters  work  against  a  head  of  14  feet.  The  apparatus  for  regu- 
lating the  rate  of  filtration  was  described  on  page  51. 

As  yet  no  full  description  of  the  Miiggel  works  has  been  pub- 


FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

lished,  but  they  resemble  closely  the  Tegel  works.  Both  were  de- 
signed by  or  under  the  direction  of  the  late  director  of  the  water- 
works, Mr.  Henry  Gill. 

The  average  daily  quantity  of  water  supplied  for  the  fiscal 
year  ending  March  31,  1893,  was  29,000,000  gallons  daily,  which  es- 
timate allows  10  per  cent  for  the  slip  of  the  pumps.  Of  this  quan- 
tity 9,650,000  was  furnished  by  Stralau  and  19,350,000  by  Tegel. 
The  greatest  consumption  in  a  single  day  was  43,300,000  gallons,  or 
26.6  gallons  per  head,  while  the  average  quantity  for  the  year  was 
18.4  gallons  per  head.  All  water  without  exception  is  sold  by  meter, 
the  prices  ranging  from  27.2  cents  a  thousand,  gallons  for  small  con- 
sumers to  13.6  cents  for  large  consumers  and  manufacturers.  The 
average  receipts  for  all  water  pumped,  including  that  used  for  pub- 
lic purposes  and  not  paid  for,  were  15.4  cents  a  thousand  gallons,, 
against  the  cost  of  production,  9.8  cents,  which  covers  operating  ex- 
penses, interest  on  capital,  and  provision  for  sinking  fund.  This 
leaves  a  handsome  net  profit  to  the  city.  On  account  of  the  com- 
paratively high  price  of  the  city  water  and  the  ease  with  which  well- 
water  is  obtained,  the  latter  is  almost  exclusively  used  for  running 
engines,  manufacturing  purposes,  etc.,  and  this  in  part  explains  the 
very  low  per-capita  consumption. 

The  volume  of  sewage,  however,  for  the  same  year,  including 
rain-water,  except  during  heavy  showers,  was  only  29  gallons  per 
head,  showing  even  with  the  private  water-supplies  an  extraordi- 
narily low  consumption. 

The  friction  of  the  water  in  the  4.75  miles  of  3-foot  pipe  between 
Tegel  and  the  reservoir  at  Charlottenburg  presents  an  interesting 
point.  When  well-water  with  crenothrix  was  pumped,  the  friction  rose 
to  34.5  feet,  when  the  velocity  was  2.46  feet  per  second.  According  to 
Herr  Anklamm,  who  had  charge  of  the  works  at  the  time,  the  friction 
was  reduced  to  19.7  feet  when  filtered  water  was  used  and  after  the 
pipe  had  been  flushed,  and  this  has  not  increased  with  continued 
use.  He  calculated  the  friction  for  the  velocity  according  to  Darcy 
15.0  feet,  Lampe  17.8  feet,  Weisbach  18.7  feet,  and  Prony  21.5  feet. 


APPENDIX    VII.  171 


APPENDIX  VII. 
ALTONA  WATER-WORKS. 

THE  Altona  water-works  are  specially  interesting  as  an  example 
of  a  water  drawn  from  a  source  polluted  to  a  most  unusual  extent : 
the  sewage  from  cities  with  a  population  of  770,000,  including  its 
own,  is  discharged  into  the  river  Elbe  within  ten  miles  above  the 
intake  and  upon  the  same  side. 

The  area  of  the  watershed  of  the  Elbe  above  Altona  is  about 
52,000  square  miles,  and  the  average  rainfall  is  estimated  to  be 
about  28  inches,  varying  from  24  or  less  near  its  mouth  to  much 
higher  quantities  in  the  mountains  far  to  the  south.  On  this  water- 
shed there  are  46  cities,  which  in  1890  had  populations  of  over  20,000 
each,  and  in  addition  there  is  a  permanent  population  upon  the  river- 
boats  estimated  at  20,000,  making  in  all  5,894,000  inhabitants,  with- 
out including  either  country  districts  or  the  numberless  cities  with 
less  than  20,000  inhabitants  each.  The  sewage  from  about  1,700,000 
of  these  people  is  purified  before  being  discharged  ;  and  assuming 
that  as  many  people  living  in  cities  smaller  than  20,000  are  con- 
nected with  sewers  as  live  in  larger  places  without  being  so  con- 
nected, the  sewage  of  over  four  million  people  is  discharged 
untreated  into  the  Elbe  and  its  tributaries. 

The  more  important  of  these  sources  of  pollution  are  the  fol- 
lowing : 

P.  Population  On  what     Approximate 

in  1890.  River.    Distance,  Miles- 

Shipping 20,000 


Altona 143,353  Elbe  6 

Hamburg 570,534  "  7 

Wandabeck 20,586  "  8 


172  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

Q  Population  On  what      Approximate 

in  1890.  River.     Distance,  Miles, 

Harburg 35>Ioi  Elbe  n 

Magdeburg   202,325               "  185 

Dresden 276,085               "  354 

Berlin  and  suburbs 1,787,859  Havel  243 

Halle 101,401  Saale  272 

Leipzig 355485  Elster  305 

Chemnitz 138,955  Mulde  340 

Prague 310,483  Moldau  500 

The  sewage  of  Berlin  and  of  most  of  its  suburbs  is  treated  before 
being  discharged,  and  in  addition  the  Havel  flows  through  a  series 
of  lakes  below  the  city,  allowing  better  opportunities  for  natural 
purification  than  in  the  case  of  any  of  the  other  cities.  Halle  treats 
less  than  a  tenth  of  its  sewage.  Magdeburg  will  treat  its  sewage  in 
the  course  of  a  few  years.  Leipzig,  Chemnitz,  and  other  places  are 
thinking  more  or  less  seriously  of  purification. 

The  number  of  bacteria  in  the  raw  water  at  Altona  fluctuates 
with  the  tide  and  is  extremely  variable;  numbers  of  50,000  and 
100,000  are  not  infrequent,  but  10,000  to  40,000  is  perhaps  about 
the  usual  range. 

The  works  were  originally  built  by  an  English  company  in  1860, 
and  have  since  been  greatly  extended.  They  were  bought  by  the 
city  some  years  ago.  The  water  is  pumped  directly  from  the  river 
to  a  settling-basin  upon  a  hill  280  feet  above  the  river.  From  this 
it  flows  by  gravity  through  the  filters  to  the  slightly  lower  pure- 
water  reservoir  and  to  the  city  without  further  pumping.  The 
filters  are  open,  with  nearly  vertical  masonry  walls,  as  described  in 
Kirkwood's  report.  The  cross-section  of  the  main  underdrain  is 

WTTO  °f  the  area  °f  t^ie  beds. 

Considerable  trouble  has  been  experienced   from  frost.     Wit 
continued  cold  weather  it    is  extremely  difficult    to   satisfactoril 
scrape  the  filters,  and  very  irregular  rates  of  filtration  may  result 
at   such  times.     In   the  last    few   years,  with   systematic  bacterial 
investigation,  it  has  been  found   that   greatly  decreased  efficiency 


1 


APPENDIX    VII.  173 

frequently  follows  continued  cold  weather,  and  the  mild  epidemics 
of  typhoid  fever  from  which  the  city  has  long  suffered  have 
generally  occurred  after  these  times.  Thus  a  light  epidemic  of 
typhoid  in  1886  came  in  March,  following  a  light  epidemic  in 
Hamburg.  In  1887  a  severe  epidemic  in  February  followed  a 
severe  epidemic  in  Hamburg  in  December  and  January.  In  1888 
a  severe  epidemic  in  March  followed  an  epidemic  in  Hamburg 
lasting  from  November  to  January.  Hamburg's  epidemic  of 
1889,  coming  in  warm  weather,  September  and  October,  was  fol- 
lowed by  only  a  very  slight  increase  in  Altona.  In  1891  Altona 
suffered  again  in  February  from  a  severe  epidemic,  although  very 
little  typhoid  had  been  in  Hamburg.  A  less  severe  outreak  also 
came  in  February,  1892,  and  a  still  slighter  one  in  February,  1893. 
In  the  ten  years  1882-1892,  of  five  well-marked  epidemics,  three 
broke  out  in  February  and  two  in  March,  while  two  smaller  out- 
breaks came  in  December  and  January.  No  important  outbreak 
has  ever  occurred  in  summer  or  in  the  fall  months,  when  typhoid  is 
usually  most  prevalent,  thus  showing  clearly  the  bad  effect  of  frost 
upon  open  filters  (see  Appendix  II).  With  steadily  increasing 
consumption  the  sedimentation-basin  capacity  of  late  years  has 
become  insufficient  as  well  as  the  filtering  area,  and  it  is  not  unlikely 
that  with  better  conditions  a  much  better  result  could  be  obtained 
in  winter  even  with  open  filters.* 

The  brilliant  achievement  of  the  Altona  filters  was  in  the  sum- 
mer of  1892,  when  they  protected  the  city  from  the  cholera  which 

*  In  the  Centralblatt  fiir  Bakteriologie,  1895,  page  881,  Reinsch  discusses  at  length 
the  cause  of  the  inferior  results  at  Altona  in  winter,  and  has  apparently  discovered  a 
new  factor  in  producing  them.  Owing  to  defective  construction  of  the  outlets  for  the 
sedimentation-basins  they  have  failed  to  act  properly  in  presence  of  excessive  quantities 
of  ice,  and  the  sediment  from  the  basins  has  been  discharged  in  large  quantity  upon 
the  filters,  and  a  small  fraction  of  the  many  millions  of  bacteria  in  it  have  passed 
through  the  filters.  He  has  experimented  with  this  sediment  applied  to  small  filters, 
and  has  become  convinced  that  to  secure  good  work  under  all  conditions  a  much  deeper 
layer  of  sand  than  that  generally  considered  necessary  must  be  used,  and  his  work 
emphasizes  the  importance  of  the  action  of  the  sand  in  distinction  from  the  action  of 
the  sediment  layer,  which  has  often  been  thought  to  be  the  sole,  or  at  least  the  princi- 
pal, requirement  of  good  filtration. 


174  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

so  ravaged  Hamburg,  although  the  raw  water  at  Altona  must  have 
contained  a  vastly  greater  quantity  of  infectious  matter  than  that 
which  worked  such  havoc  in  Hamburg. 

From  these  records  it  appears  that  for  about  nine  months  of  the 
year  the  Altona  filters  protect  the  city  from  the  impurities  of  the 
Elbe  water,  but  that  during  cold  weather,  with  continued  mean 
temperatures  below  the  freezing-point,  such  protection  is  not  com- 
pletely afforded,  and  bad  effects  have  occasionally  resulted.  Not- 
withstanding the  recent  construction  of  open  filters  in  Hamburg  it 
appears  to  me  that  there  must  always  be  more  or  less  danger  from 
open  filters  in  such  a  climate.  Hamburg's  danger,  however,  will  be 
much  less  than  Altona's  on  account  of  its  better  intake  above  the 
outlets  of  the  sewers  of  Hamburg  and  Altona,  which  are  the  most 
important  points  of  pollution  at  Altona. 


APPENDIX    VIII.  175 


APPENDIX    VIII. 
HAMBURG  WATER-WORKS. 

THE  source  and  quality  of  the  water  previously  supplied  has  been 
sufficiently  indicated  in  Appendix  II.  It  was  originally  intended 
to  filter  the  water,  but  the  construction  of  filters  was  postponed 
from  time  to  time  until  the  fall  of  1890,  when  the  project  was 
seriously  taken  up,  and  work  was  commenced  in  the  spring  of  1891. 
Three  years  were  allowed  for  construction.  In  1892,  however,  the 
epidemic  of  cholera  came,  killing  8605  residents  and  doing  incalcu- 
lable damage  to  the  business  interests  of  the  city.  The  health 
authorities  found  that  the  principal  cause  of  this  epidemic  was  the 
polluted  water-supply.  To  prevent  a  possible  recurrence  of  cholera 
in  1893,  the  work  of  construction  of  the  filters  was  pressed  forward 
much  more  rapidly  than  had  been  intended.  Electric  lights  were 
provided  to  allow  the  work  to  proceed  nights  as  well  as  days,  and 
as  a  result  the  plant  was  put  in  operation  May  27,  1893,  a  full  year 
before  the  intended  time.  Owing  to  the  forced  construction  the 
cost  was  materially  increased. 

The  new  works  take  the  raw  water  from  a  point  one  and  a  half 
miles  farther  up-stream,  where  it  is  believed  the  tide  can  never  carry 
the  city's  own  sewage,  as  it  did  frequently  to  the  old  intake. 
The  water  is  pumped  from  the  river  to  settling-basins  against  heads 
varying  with  tide  and  the  water-level  in  the  basins  from  8  to  22 
feet.  Each  of  the  four  settling-basins,  has  an  area  of  about  10 
acres,  and,  with  the  water  6.56  feet  deep,  holds  20,500,000  gallons, 
or  82,000,000  gallons  in  all.  The  works  are  intended  to  supply  a 
maximum  of  48,000,000  gallons  daily,  but  the  present  average  con- 
sumption i?  only  about  35,000,000  gallons  (1892),  or  59  gallons  per 


176  FILTRATION  OF  PUBLIC  WATER-SUPPLIES, 

head  for  600,000  population.  This  consumption  is  regarded  as  ex- 
cessive, and  it  is  hoped  that  it  will  be  reduced  materially  by  the  more 
general  use  of  meters.  The  sedimentation-basins  are  surrounded  by 
earthen  embankments  with  slopes  of  1 :  3,  the  inner  sides  being  paved 
with  brick  above  a  clay  layer.  The  water  flows  by  gravity  from 
these  basins  to  the  filters,  a  distance  of  ij-  miles,  through  a  conduit 
8£  feet  in  diameter.  The  flow  of  the  water  out  of  the  basins  and 
from  the  lower  end  of  the  conduit  is  regulated  by  automatic  gates 
connected  with  floats,  shown  by  Fig.  n,  page  56. 

The  filters  are  18  in  number,  and  each  has  an  effective  area 
of  1.89,  or  34  acres  in  all.  They  are  planned  to  filter  at  a  rate 
of  1. 60  million  gallons  per  acre  daily,  which  with  16  filters  in  use 
gives  a  daily  quantity  of  48,000,000  gallons  as  the  present  limit  of 
the  works.  The  sides  of  the  filters  are  embankments  with  I  :  2 
slopes.  Both  sides  and  bottoms  have  20  inches  of  packed  clay, 
above  which  are  4  inches  of  puddle,  supporting  a  brick  pavement 
laid  in  cement.  The  bricks  are  laid  flat  on  the  bottom,  but  edge- 
wise on  the  sides  where  they  will  come  in  contact  with  ice. 

The  main  effluent-drain  has  a  cross-section  for  the  whole  length 
of  the  filter  of  4.73  square  feet,  or  TT^or  of  the  area  of  the  filter;  and 
even  at  the  low  rate  of  filtration  proposed,  the  velocity  in  the  drain 
will  reach  0.97  foot.  The  drain  has  brick  sides,  1.80  feet  high,  cov- 
ered with  granite  slabs.  The  lateral  drains  are  all  of  brick  with 
numerous  large  openings  for  admission  of  water.  They  are  not 
ventilated,  and  I  am  unable  to  learn  that  any  bad  results  follow 
this  omission. 

The  filling  of  the  filters  consists  of  2  feet  of  gravel,  the  top 
being  of  course  finer  than  the  bottom  layers,  above  which  are  40 
inches  of  sand,  which  are  to  be  reduced  to  24  inches  by  scraping 
before  being  refilled.  The  water  over  the  sand,  when  the  latter  is  of 
full  depth,  is  43  inches  deep,  and  will  be  increased  to  59  inches  with 
the  minimum  sand-thickness.  The  apparatus  for  regulating  the  rate 
of  filtration  was  described  page  52.  The  cost  of  the  entire  plant, 
including  34  acres  effective  filter-surface,  40  acres  of  sedimentation- 


APPENDIX    VIII.  177 

basins,  over  2  miles  of  8£-foot  conduit,  pumping-machinery,  sand- 
washing  apparatus,  laboratory,  etc.,  was  about  9,500,000  marks,  or 
$2,280,000.  This  all  reckoned  on  the  effective  filter  area  is  $67,000 
per  acre,  or  $3.80  per  head  for  a  population  of  600,000. 

The  death-rate  since  the  introduction  of  filtered  water  has  been 
lower  than  ever  before  in  the  history  of  the  city,  but  as  it  is  thought 
that  other  conditions  may  help  to  this  result,  no  conclusions  are  as 
yet  drawn. 


J/8  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 


APPENDIX  IX. 
NOTES  ON  SOME  OTHER  EUROPEAN  WATER-SUPPLIES. 

Amsterdam. — The  water  is  derived  from  open  canals  in  the 
-dunes.  These  canals  have  an  aggregate  length  of  about  15  miles, 
and  drain  about  6200  acres.  The  water,  as  it  enters  the  canals 
from  the  fine  dune-sand,  contains  iron,  but  this  is  oxidized  and 
deposited  in  the  canals.  The  water  after  collection  is  filtered.  It 
has  been  suggested  that  by  using  covered  drains  instead  of  open 
canals  for  collecting  the  water,  the  filtration  would  be  unnecessary ; 
but,  on  the  other  hand,  the  cost  of  building  and  maintaining  cov- 
ered drains  in  the  very  fine  sand  would  be  much  greater  than 
that  of  the  canals,  and  it  is  believed,  also,  that  the  water  so  col- 
lected would  contain  iron,  the  removal  of  which  might  prove 
as  expensive  as  the  present  filtration.  In  1887  filters  were  built 
to  take  water  from  the  river  Vecht,  but  the  city  has  refused  to 
.allow  the  English  company  which  owns  the  water-works  to  sell 
this  water  for  domestic  purposes,  and  it  is  only  used  for  public 
and  manufacturing  purposes,  only  a  fraction  of  the  available  sup- 
ply being  required.  Leyden,  the  Hague,  and  some  other  Dutch 
cities  have  supplies  like  the  dune  supply  of  Amsterdam,  and 
they  are  invariably  filtered. 

Antwerp  is  also  supplied  by  an  English  company.  The  raw 
water  is  drawn  from  a  small  tidal  river,  which  at  times  is  polluted 
by  the  sewage  of  Brussels.  It  is  treated  by  metallic  iron  in  Ander- 
son revolver  purifiers,  and  is  afterward  filtered  at  a  rather  low 
average  rate.  The  hygienic  results  are  closely  watched  by  the  city 
authorities,  and  are  said  to  be  satisfactory. 

Rotterdam. — The  raw  water  is  drawn  from  the  Maas,  as  the 


APPENDIX  IX.  179 

Dutch  call  the  main  stream  of  the  Rhine  after  it  crosses  their 
border.  The  population  upon  the  river  and  its  tributaries  in 
Switzerland,  Germany,  Holland,  France,  and  Belgium  is  very  great ; 
but  the  flow  is  also  great,  and  the  low  water  flow  is  exception- 
ally large  in  proportion  to  the  average  flow,  on  account  of  the 
melting  snow  in  summer  in  Switzerland,  where  it  has  its  origin. 

The  original  filters  had  wooden  under-drains,  and  there  was 
constant  trouble  with  crenothrix  until  the  filters  were  recon- 
structed without  wood,  since  which  time  there  has  been  no 
farther  trouble.  The  present  filters  are  large  and  well  managed. 
There  is  ample  preliminary  sedimentation. 

Schiedam. — The  filters  at  Schiedam  are  comparatively  small, 
but  are  of  unusual  interest  on  account  of  the  way  in  which  they 
are  operated.  The  intake  is  from  the  Maas  just  below  Rotter- 
dam. The  city  was  unable  to  raise  the  money  to  seek  a  more 
distant  source  of  supply,  and  the  engineer,  H.  P.  N.  Halbertsma, 
was  unwilling  to  recommend  a  supply  from  so  doubtful  a  source 
without  more  thorough  treatment  than  simple  sand-filtration  was 
then  thought  to  be.  The  plan  adopted  is  to  filter  the  sup- 
ply after  preliminary  sedimentation  through  two  filters  of  0.265 
acre  each,  and  the  resulting  effluent  is  then  passed  through  three 
other  filters  of  the  same  size.  River  sand  is  used  for  the  first, 
and  the  very  fine  dune  sand  for  the  second  filtration.  The  cost 
both  of  construction  and  operation  was  satisfactory  to  the  city, 
and  much  below  that  of  any  other  available  source ;  and  the 
hygienic  results  have  been  equally  satisfactory,  notwithstanding 
the  unfavorable  position  of  the  intake. 

Magdeburg. — The  supply  is  drawn  from  the  Elbe,  and  is  fil- 
tered through  vaulted  filters  after  preliminary  sedimentation.  The 
pollution  of  the  river  is  considerable,  although  less  than  at  Altona 
or  even  at  Hamburg.  The  city  has  been  troubled  at  times  by 
enormous  discharges  of  salt  solution  from  salt-works  farther  up, 
which  at  extreme  low  water  have  sometimes  rendered  the  whole 
river  brackish  and  unpleasant  to  the  taste ;  but  arrangements  have 


180  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

now  been  made  which,  it  is  hoped,  will  prevent  the  recurrence  of 
this  trouble. 

Breslau  is  supplied  with  filtered  water  from  the  river  Oder, 
which  has  a  watershed  of  8200  square  miles  above  the  intake, 
and  is  polluted  by  the  sewage  from  cities  with  an  aggregate  popu- 
lation of  about  200,000,  some  of  which  are  in  Galicia,  where  cholera 
is  often  prevalent.  In  recent  years  the  city  has  been  free  from 
cholera,  and  from  more  than  a  very  limited  number  of  typhoid- 
fever  cases  ;  but  the  pollution  is  so  great  as  to  cause  some  anxi- 
ety, notwithstanding  the  favorable  record  of  the  filters,  and  there 
is  talk  of  the  desirability  of  securing  another  supply.  Until 
1893  there  were  four  filter-beds,  with  areas  of  1.03  acres  each, 
and  not  covered.  In  1893  a  fifth  bed  was  added.  This  is 
covered  by  vaulting  and  is  divided  into  four  sections,  which  are 
separately  operated,  so  that  it  is  really  four  beds  of  0.25  acre  each. 
The  vaulting  is  concrete  arches,  supported  by  steel  I  beams  in 
one  direction. 

Budapest. — A  great  variety  of  temporary  water-supplies  have 
at  different  times  been  used  by  this  rapidly  growing  city.  The 
filters  which  for  some  years  have  supplied  a  portion  of  the  supply 
have  not  been  altogether  satisfactory  ;  but  perhaps  this  was  due 
to  lack  of  preliminary  sedimentation  for  the  extremely  turbid 
Danube  water,  and  also  to  inadequate  filter-area.  The  city  is 
rapidly  building  and  extending  works  for  a  supply  of  ground- 
water,  and  in  1894  the  filters  were  only  used  as  was  necessary  to 
supplement  this  supply,  and  it  was  hoped  that  enough  well-water 
would  be  obtained  to  allow  the  filters  to  be  abandoned  in  the  near 
future.  The  Danube  above  the  intake  receives  the  sewage  of 
Vienna  and  innumerable  smaller  cities,  but  the  volume  of  the  river 
is  very  great  compared  to  other  European  streams,  so  that  the  rela- 
lative  pollution  is  not  so  great  as  in  many  other  places. 

Zurich. — The  raw  water  is  drawn  by  the  city  from  the  Lake 
of  Zurich  near  its  outlet,  and  but  a  few  hundred  feet  from  the 
heart  of  the  city.  Although  no  public  sewers  discharge  into  the 


APPENDIX  IX. 

lake,  there  is  some  pollution  from  boats  an^tiaUiU'S^and  other 
sources,  and,  judging  by  the  number  of  bacteria  in  the  raw  water, 
this  pollution  is  increasing.  The  raw  water  is  extremely  free 
from  sediment,  and  the  filters  only  become  clogged  very  slowly. 
The  rate  of  filtration  is  high,  habitually  reaching  7,000,000  gal- 
lons per  acre  daily ;  but,  with  the  clear  lake  water  and  long 
periods  between  scrapings,  the  results  are  excellent  even  at  this 
rate.  The  filters  are  all  covered  with  concrete  groined  arches. 

Filtration  was  commenced  in  1886,  and  was  followed  by  a 
sharp  decline  in  the  amount  of  typhoid  fever,  which,  up  to  that 
time,  had  been  rather  increasing;  for  the  six  years  before  the 
change  there  were  sixty-nine  deaths  from  this  cause  annually  per 
100,000  living,  and  for  the  six  years  after  only  ten,  or  one  seventh 
as  many ;  and  this  reduction  is  attributed  by  the  local  authori- 
ties to  the  filtration.* 

St.  Petersburg. — The  supply  is  drawn  from  the  Neva  River  by 
an  English  company,  and  is  filtered  through  vaulted  filters  at  a 
very  high  rate. 

Warsaw. — The  supply  is  drawn  from  the  Weichsel  River  by 
the  city,  and  is  filtered  through  vaulted  filters  after  preliminary 
sedimentation  at  a  rate  never  exceeding  3,570,000  gallons  per 
acre  daily. 

THE  USE  OF  UNFILTERED   SURFACE-WATERS. 

The  use  of  surface-water  without  filtration  in  Europe  is  com- 
paratively limited.  In  Germany  this  use  is  now  prohibited  by  the 
Imperial  Board  of  Health.  In  Great  Britain,  Glasgow  draws  its 
supply  unfiltered  from  Loch  Katrine ;  and  Manchester  and  some 
other  towns  use  unfiltered  waters  from  lakes  or  impounding  reservoirs 
the  watersheds  of  which  are  entirely  free  from  population.  The  best 
English  practice,  however,  as  in  Germany,  requires  the  filtration  of 
such  waters  even  if  they  are  not  known  to  receive  sewage,  and  the 

*Licht-u.  Wasservverke,  Zurich,  1892,  page  32. 


1 82  FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 

unpolluted  supplies  of  Liverpool,  Bradford,  Dublin,  and  many  other 
cities  are  filtered  before  use. 

THE  USE  OF  GROUND-WATER.* 

Ground-waters  are  extensively  used  in  Europe,  and  apparently  in 
some  localities  the  geological  formations  are  unusually  favorable  to 
this  kind  of  supply.  Paris  derives  all  the  water  it  now  uses  for  do- 
mestic purposes  from  springs,  but  has  a  supplementary  supply  from 
the  river  for  other  purposes.  Vienna  and  Munich  also  obtain  their 
entire  supplies  from  springs,  while  Budapest,  Cologne,  Leipzig,  Dres- 
den, Frankfurt,  many  of  the  great  French  cities,  Brussels,  a  part  of 
London,  and  many  other  English  cities  derive  their  supplies  from 
wells  or  filter-galleries,  and  among  the  smaller  cities  all  over  Europe 
ground-water  supplies  are  more  numerous  than  other  kinds. 

*  Descriptions  of  some  of  the  leading  European  ground-water  supplies  were  given 
by  the  author  in  the  Jour.  Asso.  Eng.  Soc.,  Feb.  1895,  p.  113. 


APPENDIX  X.  183 


APPENDIX  X. 
LITERATURE  OF    FILTRATION. 

THE  following  is  a  list  of  a  number  of  articles  on  filtration.  The 
list  is  not  complete,  but  it  is  believed  that  it  contains  the  greater  part 
of  articles  upon  slow  sand-filtration,  and  that  it  will  prove  serviceable 
to  those  who  wish  to  study  the  subject  more  in  detail. 

ANKLAMM.     Glasers  Annalen,  1886,  p.  48. 

A  description  of  the  Tegel  filters  at  Berlin,  with  excellent  plans. 
BAKER.     Engineering  News. 

Water  purification  in  America:  a  series  of  descriptions  of  filters, 
as  follows  :  Aug.  3,  1893,  Lawrence  filter  and  description  of  appara- 
tus of  screening  sand  and  gravel;  Apr.  26,  1894,  filter  at  Nantucket, 
Mass.;  June  7,  1894,  filters  at  Ilion,  N.  Y.,  plans;  June  14,  1894, 
filters  at  Hudson,  N.  Y.;  July  12,  1894,  filters  at  Zurich,  Switzerland, 
plans  ;  Aug.  23,  1894,  filters  at  Mt.  Vernon,  N.  Y.,  plans. 
BERTSCHINGER.  Journal  fiir  Gas-  und  Wasserversorgung,  1889,  p.  1126. 
A  record  of  experiments  made  at  Zurich  upon  the  effect  of  rate 
Qf  filtration,  scraping,  and  the  influence  of  vaulting.  Rate  and  vault- 
ing were  found  to  be  without  effect,  but  poorer  results  followed  scrap- 
ing. The  numbers  of  bacteria  in  the  lake-water  were  too  low  to 
allow  conclusive  results. 

Journal  fiir  Gas-  und  Wasserversorgung,  1891,  p.  684. 

A  farther  account  of  the  Zurich  results,  with  full  analyses  and 
a  criticism  of  Frankel  and  Piefke's  experiments. 
BOLTON.     Pamphlet,  1884. 

Descriptions  and  statistics  of  London  filters. 
BOTTCHER  and  OHNESORGE.     Zeitschrift  fiir  Bauwesen,  1876,  p.  343. 

A  description  of  the  Bremen  works,  with  full  plans. 
BURTON.    Water-supply  of  Towns.     London,  1894. 

Pages  94-115  are  upon  filtration  and  mention  a  novel  method 
of  regulating  the  rate. 
CODD.     Engineering  News,  Apr.  26,  1894. 

A  description  of  a  filter  at  Nantucket,  Mass. 


1 84  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

CRAMER.     Centralblatt  fiir  Bauwesen,  1886,  p.  42. 

A  description  of  filters  built  at  Brieg,  Germany. 
CROOK.     London  Water-supply.     London,  1883. 
DELBRUCK.     Allgemeine  Bauzeitung,  1853,  p.  103. 

A  general  article  on  filtration  ;  particularly  valuable  for  notices 
of  early  attempts  at  filtration  and  of  the  use  of  alum. 
Deutsche  Verein  von  Gas-  und  Wasserfachmanner. 

Stenographic  reports  of  the  proceedings  of  this  society  are  printed 
regularly  in  the  Journal  fur  Gas-  und  Wasserversorgung,  and  the 
discussions  of  papers  are  often  most  interesting. 
DROWN.     Journal  Association  Eng.  Societies,  1890,  p.  356. 

Filtration  of  natural  waters. 
FISCHER.     Vierteljahresschrift  fiir  Gesundheitspflege,  1891,  p.  82. 

Discussion  of  papers  on  water-filtration. 
FRANKEL.     Vierteljahresschrift  fiir  Gesundheitspflege,  1891,  p.  38. 

On  filters  for  city  water-works. 
FRANKEL  and   PIEFKE.     Zeitschrift  fiir  Hygiene,  1891,  p.  38,  Leistungen 

der  Sandfiltern. 

E.  FRANKLAND.     Report  in  regard  to  the  London  filters  for  1893  in  the 
Annual  Summary  of  Births,  Deaths,  and  Causes  of  Death  in  London 
and  Other   Great   Towns,    1893.      Published   by   authority   of   the 
Registrar-  Gen  eral. 
P.  FRANKLAND.     Proc.  Royal  Society,  1885,  p.  379. 

The  removal  of  micro-organisms  from  water. 

Proceedings  Inst.  Civil  Engineers,  1886,  Ixxxv.  p.  197. 

Water-purification  ;  its  biological  and  chemical  basis. 

Trans,  of  Sanitary  Institute  of  Great  Britain,  1886. 

Filtration  of  water  for  town  supply. 
FRUHLING.     Handbuch  der  Ingenieurwissenschaften,  vol.  ii. 

Chapter  on  water-filtration  gives  general  account  of  filtration, 
with  details  of  Konigsberg  filters  built  by  the  author  and  not  else- 
where published. 
FULLER.     Report  Mass.  State  of  Board  of  Health,  1892,  p.  449. 

"      "        "       "       "         1893,  p.  453- 

Accounts  of  the  Lawrence  experiments  upon  water-filtration  for 
1892  and  1893. 

American  Public  Health  Association,  1893,  p.  152. 

On   the   removal  of  pathogenic  bacteria   from   water   by  sand 
filtration. 

American  Public  Health  Association,  1894,  p.  64. 

Sand  filtration  of  water  with  special  reference  to  results  obtained 
at  Lawrence,  Mass. 


APPENDIX  X.  185 

GILL.     Deutsche  Bauzeitung,  1881,  p.  567. 

On  American  rapid  filters.  The  author  shows  that  they  are  not 
to  be  thought  of  for  Berlin,  as  they  would  be  more  expensive  as  well 
as  probably  less  efficient  than  the  usual  procedure. 

Journal  fiir  Gas-  und  Wasserversorgung,  1892,  p.  596. 

A  general  account  of  the  extension  of  the  Berlin  filters  at  Mug- 
gel.  No  drawings. 

GRAHN.     Journal  fiir  Gas-  und  Wasserversorgung,  1877,  p.  543. 
On  the  filtration  of  river-waters. 

Journal  fiir  Gas-  und  Wasserversorgung,  1890,  p.  511. 

Filters  for  city  water-works. 

Vierteljahresschrift  fiir  Gesundsheitpflege,  1891,  p.  76. 

Discussion  of  papers  presented  on  filtration. 

Journal  fiir  Gas-  und  Wasserversorgung,  1894,  p.  185. 

A  history  of  the  "  Rules  for  Water-filtration  "  (Appendix  I),  with 
some  discussion  of  them. 

GRAHN  and  MEYER.     Reiseberichte  iiber  kiinstliche  central  Sandfiltra- 
tion.     Hamburg,  1876. 

An  account  of  the  observations  of  the  authors  in  numerous  cities, 
especially  in  England. 
GRENZMER.     Centralblatt  der  Bauverwaltung,  1888,  p.  148. 

A  description  of  new  filters  at  Amsterdam,  with  plans. 
GRUBER.     Centralblatt  fiir  Bacteriologie,  1893,  p.  488. 

Salient  points  in  judging  of  the  work  of  sand-filters. 
HALBERTSMA.     Journal  fiir  Gas-  und  Wasserversorgung,  1892,  p.  43. 

Filter-works  in  Holland.  Gives  sand,  gravel,  and  water  thick- 
ness, with  diagrams. 

Journal  fiir  Gas-  und  Wasserversorgung,  1892,  p.  686. 

Description  of  filters  built  by  the  author  at  Leeuwarden,  Hol- 
land, with  plans. 
HART.     Proceedings  Inst.  of  Civil  Engineers,  1890,  c.  p.  217. 

Description  of  filters  at  Shanghai. 
HAUSEN.     Journal  fiir  Gas-  und  Wasserversorgung,  1892,  p.  332. 

An   account  of  experiments  made  for  one  year  with  three  16- 
inch  filters  at  Helsingfors,  Finland,  with-weekly  analyses  of  effluents. 
HAZEN.     Report  of  Mass.  State  Board  of  Health,  1891,  p.  601. 
Experiments  upon  the  filtration  of  water. 

Report  of  Mass.  State  Board  of  Health,  1892,  p.  539. 

Physical  properties  of  sands  and  gravels  with  reference  to  their 
use  in  filtration.     '(Appendix  III.) 
HUNTER.     Engineering,  1892,  vol.  53,  p.  621. 

Description  of  author's  sand-washing  apparatus. 


1 86  FILTRATION  OF  PUBLIC  WATER-SUPPLIES. 

1 
KIRKWOOD.     Filtration  of  River-water.     New  York,  1869. 

A  report  upon  European  filters  for  the  St.  Louis  Water  Board  in 
1866.  Contains  a  full,  account  of  thirteen  filtration-works  visited  by 
the  author,  and  of  a  number  of  filter-galleries,  with  a  project  for  filters 
for  St.  Louis.  This  project  was  never  executed,  but  the  report  is  a 
wonderful  work  which  appeared  a  generation  before  the  American 
public  was  able  to  appreciate  it.  It  was  translated  into  German,  and 
the  German  edition  was  widely  circulated  and  known. 
KOCH.  Zeitschrift  fiir  Hygiene,  1893. 

Water-filtration    and  Cholera :    a   discussion   of  the    Hamburg 
epidemic  of  1892  in  reference  to  the  effect  of  filtration. 
KROHNKE.     Journal  fiir  Gas-  und  Wasserversorgung,  1893,  p.  513. 

An  account  of  experiments  made  at  Hamburg,  as  a  result  of 
which  the  author  recommends  the  addition  of  cuprous  chloride  to 
the  water  before  filtration  to  secure  greater  bacterial  efficiency. 
KUMMEL.     Journal  fiir  Gas-  und  Wasserversorgung,  1877,  p.  452. 
Operation  of  the  Altona  filters,  with  analyses. 

Vierteljahresschrift  fiir  Gesundheitspflege,  1881,  p.  92. 

The  water-works  of  the  city  of  Altona. 

Journal  fiir  Gas-  und  Wasserversorgung,  1887,  p.  522. 

An  article  opposing  the  use  of  rapid  filters  (David's  process). 

Journal  fiir  Gas-  und  Wasserversorgung,  1890,  p.  531. 

A  criticism  of  Frankel  and  Piefke's  results,  with  some  statistics 
of  German  and  English  filters.  (The  English  results  are  taken  with- 
out credit  from  Kirkwood.) 

• Vierteljahresschrift  fiir  Gesundheitspflege,  1891,  p.  87. 

Discussion  of  papers  on  filtration,  with  some  statistics. 

Vierteljahresschrift  fiir  Gesundheitspflege,  1892,  p.  385. 

The  epidemic  of  typhoid-fever  in  Altona  in  1891. 

• Journal  fiir  Gas-  und  Wasserversorgung,  1893,  p.  161. 

Results    of   experiments  upon  filtration  made  at  Altona,   and 
bacterial   results   of   the   Altona  filters  in  connection  with  typhoi 
death-rates. 

Trans.  Am.  Society  of  Civil  Engineers,  1893,  xxx.  p.  330. 

Questions  of  water-filtration. 
LESLIE.     Trans.  Inst.  Civil  Engineers,  1883,  Ixxiv.  p.  no. 

A  short  description  of  filters  at  Edinburgh. 

LINDLEY.     A  report  for  the  commissioners  of  the  Paris  Exposition  oi 

1889  upon  the  purification  of  river-waters,  and  published  in  French 

or  German  in  a  number  of  journals,  among  them  Journal  fiir  Gas- 

und  Wasserversorgung,  1890,  p.  501. 


APPENDIX  X.  IS/ 

This  is  a  most  satisfactory  discussion  of  the  conditions  which 
modern  experience  has  shown  to  be  essential  to  successful  nitration. 
MASON.     Engineering  News,  Dec.  7,  1893. 

Filters  at  Stuttgart,  Germany,  with  plans. 
MEYER  and  SAMUELSON.     Deutsche  Bauzeitung,  1881,  p.  340. 

Project  for  niters    for  Hamburg,  with    diagrams.      Except  in 
detail,  this  project  is  the  same  as  that  executed  twelve  years  later. 
MEYER.     Deutsche  Bauzeitung,  1892,  p.  519. 

Description  of  the  proposed  Hamburg  filters,  with  diagrams. 
The  Water-works  of  Hamburg. 

A  paper  presented  to  the  International  Health  Congress  at 
Rome,  March  1894,  and  published  as  a  monograph.  It  contains  a 
full  description  of  the  filters  as  built,  with  drawings  and  views  in 
greater  detail  than  the  preceding  paper. 

MILLS.     Special  Report  Mass.  State  Board  of  Health  on  the  Purification 
of  Sewage  and  Water,  1890,  p.  60 1. 

An  account  of  the  Lawrence  experiments,  1888-1890. 
Report  Mass.  State  Board  of  Health,  1893,  p.  543. 

The  Filter  of  the  Water-supply  of  the  City  of  Lawrence  and  its 
Results. 
Trans.  Am.  Society  of  Civil  Engineers,  1893,  xxx.  p.  350. 

Purification  of  Sewage  and  Water  by  Filtration. 
NEVILLE.     Engineering,  1878,  xxvi.  p.  324. 

A  description  of  the  Dublin  filters,  with  plans. 
NICHOLS.     Report  Mass.  State  Board  of  Health,  1878,  p.  137. 

The  filtration  of  potable  water. 
OESTER.     Gesundheits-Ingenieur,  1893,  p.  505. 

What  is  the  Rate  of  Filtration  ?     A  purely  theoretical  discussion. 
ORANGE.     Trans.  Inst.  Civil  Engineers,  1890,  c.  p.  268. 

Filters  at  Hong  Kong. 
PFEFFER.     Deutsche  Bauzeitung,  1880,  p.  399. 

A  description  of  filters  at  Liegnitz,  Germany. 
PIEFKE.      Results   of   Natural   and   Artificial   Filtration.      Berlin,   1881. 

Pamphlet. 
Journal  fur  Gas-  und  Wasserversorgung,  1887,  p.  595.     Die  Prin- 

cipien  der  Reinwassergewinnung  vermittelst  Filtration. 

A  sketch  of  the  theory  and  practical  application  of  filtration. 
Zeitschrift  fiir  Hygiene,  1889,  p.  128.     Aphorismen  iiber  Wasser- 
versorgung. 

A  discussion  of  the  theory  of  filtration,  with  a  number  of  ex- 
periments on  the  thickness  of  sand-layers,  etc. 


1 88  FIL  TRA  TION  OF  P  UBLIC  WA  TER-S UPPLIES. 

PIEFKE.     Vierteljahresschrift  fiir  Gesundheitspflege,  1891,  p.  59. 

On  filters  for  city  water-works. 
FRANKEL  and  PIEFKE.     Zeitschrift  fiir  Hygiene,  1891,  p.  38. 

Leistungen  der  Sandfiltern.  An  account  of  the  partial  obstruc- 
tion of  the  Stralau  filters  by  ice,  and  a  typhoid  epidemic  which 
followed.  Experiments  were  then  made  upon  the  passage  of  cholera 
and  typhoid  germs  through  small  filters. 

PIEFKE.     Journal  fiir  Gas-  und  Wasserversorgung,  1891,  p.  208.     Neue 
Ermittelungen  iiber  Sandfiltration. 

The  above  mentioned  experiments  being  objected  to  on  certain 
grounds,  they  were  repeated  by  Pief  ke  alone,  confirming  the  previous 
observations  on  the  passage  of  bacteria  through  filters,  but  under 
other  conditions. 

Zeitschrift  fiir  Hygiene,  1894,  p.  151.     Uber  Betriebsfiihrung  von 

Sandfiltern. 

A  full  account  of  the  operation  of  the  Stralau  filters  in  1893, 
with  discussion  of  the  efficiency  of  filtration,  etc. 
PLAGGE  AND  PROSKAUER.     Zeitschrift  fiir  Hygiene,  n.  p.  403. 

Examination  of  water  before  and  after  filtration  at  Berlin,  with 
theory  of  filtration. 

REINCKE.     Bericht  iiber  die  Medicinische  Statistik  des  Hamburgischen 
Staates  fiir  1892. 

Contains  a  most  valuable  discussion  of  the  relations  of  filtration 
to  cholera,  typhoid  fever,  and  diarrhoea,  with  numerous  tables  and 
charts.     (Abstract  in  Appendix  II.) 
REINSCH.     Centralblatt  fiir  Bakteriologie,  1895,  p.  881. 

An  account  of  the  operation  of  the  Altona  filters.  High  num- 
bers of  bacteria  in  the  effluents  have  often  resulted  from  the  discharge 
of  sludge  from  the  sedimentation-basins  onto  the  filters,  due  to  the 
interference  of  ice  on  the  action  of  the  floating  outlet  for  the  basins, 
and  this,  rather  than  the  direct  effect  of  cold,  is  believed  to  be  the 
direct  cause  of  the  low  winter  efficiency.  The  author  urges  the 
necessity  of  a  deeper  sand-layers  in  no  case  less  than  18  inches  thick. 
RENK.  Gesundheits-Ingenieur,  1886,  p.  54. 

Uber  die  Ziele  der  kiinstliche  Wasserfiltration. 

RUHLMANN.     Wochenblatt  fiir  Baukunde,  1887,  p.  409. 

A  description  of  filters  at  Zurich. 
SALBACH.     Glaser's  Annalen,  1882. 

Filters  at  Groningen,  Holland,  built  in  1880.     Alum  used. 
SAMUELSON.     Translation  of  Kirkwood's  "  Filtration   of   River-waters " 
into  German,  with  additional  notes  especially  on  the  theory  of  filtra-    , 
tion  and  the  sand  to  be  employed.     Hamburg,  1876. 


APPENDIX  X.  189 

SAMUELSON.     Filtration  and   constant  water-supply.     Pamphlet.     Ham- 

burg, 1882. 
-  Journal  f.  Gas-  und  Wasserversorgung,  1892,  p.  660. 

A  discussion  of  the  best  materials  and  arrangement  for  sand-filters. 
SCHMETZEN.     Deutsche  Bauzeitung,  1878,  p.  314. 

Notice   and  extended   criticism   of   Samuelson's  translation  of 
Kirkwood. 
SEDDEN.     Jour.  Asso.  Eng.  Soc.,  1889,  p.  477. 

In  regard  to  the  sedimentation  of  river-waters. 
SEDGWICK.     New  England  Water-works  Association,  1892,  p.  103. 

European  methods  of  Filtration  with  Reference  to  American 
Needs. 
SOKAL.     Wochenschrift  der  ostreichen  Ingenieur-Verein,  1890,  p.  386. 

A  short  description  of  the  filters  at  St.  Petersburg,  and  a  com- 
parison with  those  at  Warsaw. 
STURMHOFEL.     Zeitschrift  f.  Bauwesen,  1880,  p.  34. 

A  description  of  the  Magdeburg  filters,  with  plans. 
TOMLINSON.     American  Water-works  Association,  1888. 

A  paper  on  filters  at  Bombay  and  elsewhere. 
TURNER.     Proc.  Inst.  Civil  Engineers,  1890,  c.  p.  285. 

Filters  at  Yokohama. 
VAN   DER  TAK.     Tijdschrift   van   de   Maatschapping  van  Bouwkunde, 


A  description  (in  Dutch)  of  the  Rotterdam  water-works,  includ- 
ing the  wooden  drains  which  caused  the  trouble  with  crenothrix, 
and  which  have  since  been  removed.  Diagrams. 

VAN  IJSSELSTEYN.     Tijdschrift  van  het  Koninklijk  Instituut  van  Ingen- 
ieurs,  1892-5,  p.  173. 

A  description  of  the  new  Rotterdam  filters,  with  full  drawings. 
VEITMEYER.     Verhandlungen  d.  polyt.  Gesell.  zu  Berlin,  April,  1880. 

Filtration  and  purification  of  water. 

WOLFFHUGEL.     Arbeiten  aus  dem  Kaiserliche  Gesundheitsamt,  1886,  p.  i. 
Examinations  of  Berlin  water  for  1884-5,  with  remarks  showing 
superior  bacterial  efficiency  with  open  filters. 
-  Journal  fur  Gas-  u.  Wasserversorgung,  1890,  p.  516. 

On  the  bacterial  efficiency  of  the  Berlin  filters,  with  diagrams. 
ZOBEL.     Zeitschrift  des  Vereins  deutsche  Ingenieure,  1884,  p.  537. 
Description  of  filters  at  Stuttgart. 


FILTRATION  OF  PUBLIC   WATER-SUPPLIES. 


OTHER  LITERATURE. 

Many  scientific  and  engineering  journals  publish  from  time  to 
time  short  articles  or  notices  on  filtration  which  are  not  included  in 
the  above  list.  Among  such  journals  none  gives  more  attention  to 
filtration  than  the  Journal  fur  Gasbeleuchtung  und  Wasserversorgung, 
which  publishes  regularly  reports  upon  the  operation  of  many  Ger- 
man filters,  and  gives  short  notices  of  new  construction.  The  first 
articles  upon  filtration  in  this  journal  were  a  series  of  descriptions  of 
German  water-works  in  1870-73,  including  descriptions  of  filters  at 
Altona,  Brunswick,  Liibeck,  etc.  Stenographic  reports  of  many 
scientific  meetings  have  been  published,  particularly  since  1890,  and 
since  1892  there  has  been  much  discussion  in  regard  to  the  "  Rules 
for  Filtration  "  given  in  Appendix  I. 

A  Report  of  a  Royal  Commission  to  inquire  into  the  water- 
supply  of  the  metropolis,  with  minutes  of  evidence,  appendices,  and 
maps  (London,  1893-4),  contains  much  valuable  material  in  regard 
to  filtration. 

The  monthly  reports  of  the  water  examiner,  and  other  papers 
published  by  the  Local  Government  Board,  London,  are  often  of 
interest. 

The  German  "Verein  von  Gas-  u.  Wasserfachmanner "  prints 
without  publishing  a  most  useful  annual  summary  of  German  water- 
works statistics  for  distribution  to  members.  Many  of  the  statistics 
given  in  this  volume  are  from  this  source. 

Description  of  the  filters  at  Worms  was  given  in  the  Deutsche 
Bauzeitung,  1892,  p.  508  ;  of  the  filters  at  Liverpool  in  Engineering, 
1889,  p.  152,  and  1892,  p.  739.  The  latter  journal  also  has  given  a 
number  of  descriptions  of  filters  built  in  various  parts  of  the  world 
by  English  engineers,  but,  excepting  the  articles  mentioned  in  the 
above  list,  the  descriptions  are  not  given  in  detail. 


APPENDIX  XI. 


APPENDIX  XI. 
y 

PLANS  OF  PROPOSED  FILTERS  FOR  PURIFYING  WATER  FROM 
THE  MERRIMAC  RIVER  FOR  THE  USE  OF  THE  BOSTON 
METROPOLITAN  DISTRICT. 

THESE  plans,  reproduced  in  the  accompanying  four  plates,  were 
prepared  by  the  author  in  the  fall  of  1894  as  a  part  of  a  scheme 
for  utilizing  water  from  the  Merrimac  River  for  an  additional 
supply  for  Boston  and  its  suburbs,  which  now  constitute  the 
Metropolitan  Water  District.  The  project  was  to  pump  water 
from  the  Merrimac  River  above  Lowell  to  reservoirs  a  short  dis- 
tance from  the  river,  from  which  it  would  flow  through  an  aque- 
duct, following  closely  the  line  of  the  old  Middlesex  Canal,  to 
a  point  in  the  town  of  Wilmington,  where  the  filters  shown  by 
these  plans  were  to  be  located.  The  filtered  water  was  to  flow 
through  an  aqueduct  to  a  pumping  station  at  East  Woburn,  and 
then  be  pumped  into  Spot  Pond,  which  would  serve  as  a  dis- 
tributing reservoir  for  the  water.  The  project,  including  the 
capitalized  operating  expenses,  was  found  to  be  only  ten  per 
cent  cheaper  than  a  gravity  supply  from  practically  unpolluted 
sources,  and  the  latter  was  consequently  adopted,  and  the  filters 
were  not  and  will  not  be  constructed. 

The  plans  are  reproduced  here  as  showing  perhaps  better  than 
anything  else  available  the  general  design  and  arrangement  of  a 
plant  for  purifying  water  by  sand-filtration  upon  a  very  large  scale 
and  in  a  climate  so  severe  as  to  make  covering  necessary. 

In   Plate   I   is   given  a  general  plan  of  the  whole  80  acres  of 


FILTRATION  OF  PUBLIC    WATER-SUPPLIES. 

filtering  area,  together  with  sections  of  the  outside  and  cross  walls, 
and  the  pure  water  carriers  and  drains.  The  raw  water  was  to 
liave  been  introduced  through  the  middle  and  on  each  side,  while 
the  pure  water  was  to  have  been  taken  out  by  suitable  carriers  in 
the  two  roadways  midway  between  the  middle  and  the  sides. 
These  two  carriers  united  directly  below  to  form  an  aqueduct  lead- 
ing to  the  pumping  station. 

Plate  II  is  a  ground-plan  of  one  filter,  showing  the  arrangement 
of  the  underdrainage,  inlet  and  outlet,  sand  run  and  carriers. 

In  Plate  III  are  shown  sections  of  the  vaulting  in  both  direc- 
tions; also  sections  at  the  sand  run  and  regulator  chamber,  where 
a  small  area  of  vaulting  is  raised  to  give  the  necessary  head  room. 
These  sections  also  show  the  depth  of  the  gravel,  sand,  and 
water. 

In  Plate  IV  are  shown  in  more  detail  the  regulating  devices  for 
controlling  the  level  of  the  water  on  the  filters,  and  for  maintain- 
ing the  rate  of  filtration  at  any  desired  velocity. 

The  vaulting  shown  by  the  plans  is  of  a  type  which  has  been 
successfully  used  in  covered-reservoir  construction  in  Massachu- 
setts and  elsewhere.  The  filters  shown  by  the  plans  have  an 
effective  filtering  area  of  one  acre  each,  or  80  acres  in  all.  A  part 
of  the  plant  was  to  have  been  constructed  at  once,  and  the  re- 
mainder was  to  have  been  added  from  time  to  time  as  increased 
consumption  required.  The  filtering  material  was  to  have  been 
secured  from  the  material  excavated  for  the  filters,  the  location 
having  been  selected  with  reference  to  this  in  an  extensive  sand 
and  gravel  bank.  The  material  would  require  to  be  screened  and 
:some  of  it  washed.  The  filters  were  to  be  ventilated  by  a  door  at 
the  head  of  the  sand-run,  and  by  a  large  opening  over  the  inlet 
apparatus.  For  the  purpose  of  cleaning  they  were  to  be  lighted 
with  electricity  throughout,  which  would  allow  cleaning  to  proceed 
day  and  night  if  necessary. 


APPENDIX  XL 

It  was  calculated  that  it  would  always  be  possible  to  maintain 
nine  tenths  of  the  filtering  area  in  service,  and,  at  a  rate  of  filtra- 
tion of  2.40  meters,  or  2.57  million  gallons  per  acre  daily,  80 
acres  would  yield  185  million  gallons  daily.  It  was  believed,  how- 
ever, that  at  times  of  extreme  consumption  it  would  be  possible 
to  increase  the  rate  of  filtration  somewhat,  and  the  underdrains 
and  outlet  conduits  were  calculated  throughout  to  remove  the 
water  from  nine  tenths  of  the  filters  at  a  rate  of  4  million  gallons 
per  acre  daily,  or  288  million  gallons  for  80  acres.  The  masonry 
required  for  the  work  shown  by  the  plans  is  as  follows: 

Brick  masonry  in  walls  and  vaulting. . .    1 15,000  cu.  yds. 

Rubble  masonry,  outside  walls  and  foundations..     41,000  "  " 

Concrete  in  floors  and  above  vaulting 179,000  "  " 

Total  excavation,  sand  and  soil 920,000  "  " 

Screening  sand  and  gravel. 480,000  "  " 

Washing  sand 220,000  " 

Soil  and  waste  placed  above  vaulting  or  wasted..  408,000  "  " 

Gravel  and  sand  placed  in  position  in  filters 510,000  "  " 

In  addition  to  the  above,  drain-pipes,  regulators,  gates,  sand- 
washing  machinery,  pumping  and  electric-light  plants,  labora- 
tories, etc.,  were  included  in  the  estimates.  The  total  estimated 
cost  of  filters  with  an  area  of  80  acres,  exclusive  of  land,  was 
$4,293,000,  or  $53,000  per  acre  of  effective  filtering-surface. 

In  some  small  filters  constructed  in  1895  at  Ashland,  Wis- 
consin, designed  by  Mr.  William  Wheeler,  M.  Am.  Soc.  C.  E.,  a 
groined-arch  vaulting  was  used,  which  is,  for  filters  at  least,  more 
convenient  and  satisfactory  than  the  lintel-arch  construction  shown 
by  these  plans.  In  this  design  two  layers  of  brick,  placed  flat, 
were  used,  backed  with  concrete  and  covered  with  soil.  Light  and 
ventilation  are  secured  by  circular  openings  at  the  top  of  each 
arch.  These  arches  were  constructed  by  ordinary  masons  without 


FILTRATION   OF  PUBLIC    WATER-SUPPLIES. 

difficulty,  and  have  proved  stable  and  satisfactory.  Mr.  Wheeler 
has  since  used  a  similar  design  for  covered  filters  at  Somersworth, 
New  Hampshire,  but  in  this  case  has  used  one  thickness  of  brick 
placed  on  edge,  making  the  brickwork  only  four  inches  thick, 
instead  of  five  as  at  Ashland. 


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INDEX. 


Altona,  freedom  of,  from  cholera I4g 

water-supply  of I46>  I?I 

Alum,  use  of  in  filtration gg>  IO9 

objections  to  the  use  of IIO 

Alumina,  precipitated,  use  of gg^  II3 

American  cities,  water-supplies  of,  and  typhoid  fever  in .   I26 

Amsterdam,  filters  at j~g 

Anderson  process II4>  I7g 

Antwerp,  filters  at x^g 

use  of  alum  at , IIO 

Area  of  filters  to  be  provided 43,  I42 

Bacteria,  apparent  and  actual  removal  of,  by  filters g^ 

from  underdrains 83 

in  Elbe  at  Altona I72 

in  faeces , 133 

in  water 80,  134 

method  of  determining,  in  water 140 

number  to  be  allowed  in  filtered  water 140 

of  cholera  in  river  water 149 

of  typhoid  fever,  life  of,  in  water 134 

of  special  kinds  to  test  efficiency  of  filtration 82 

to  be  determined  daily 140 

Bacterial  efficiency  of  mechanical  filters 107,  in 

examination  of  water 89,  140 

Berlin,  apparatus  for  regulating  depth  of  water  on  filters 55 

cholera  infantum  due  to  imperfectly  filtered  water 147 

friction  in  underdrains  of  filters 40 

regulation  of  rate  of  filtration 49,  5 1 

water-works 167 

Boston,  protection  of  purity  of  water-supply ...   104 

water-works,  experimental  filters 69 

Bottoms  of  filters  must  be  water-tight 143 

Breslau,  filters  at 180 

Brussels,  ground-water  supply  of 182 

Budapest,  filters  at 180 

Burton,  regulation  of  rate  at  Tokio,  Japan 54 

191 


192  INDEX. 

PAGES 

Chemnitz,  intermittent  nitration  at 100 

Chicago,  reduced  death-rate  with  new  water  intake 135 

Cholera  infantum  due  to  impure  water 144,  173 

Cholera,  in  Hamburg  due  to  water 148 

removal  of  danger  from 122 

Clark,  H.  W.,  sand  analyses  made  by. 22 

Clark's  process  for  softening  water 88 

Cleaning  niters 64 

Cologne,  water-supply  of,  from  wells 182 

Coloring  matter,  removal  of,  by  nitration 88 

Continuous  niters 5 

filtration,  nature  of  process 88 

Cost  of  niters  and  filtration..- _ 4,  16,  118,  176 

effect  of  rate  upon 44,  121 

Cost  of  Lawrence  filter 98 

Cost  of  operation  of  filters 121 

Covered  filters,  efficiency  of 17 

Covers  for  filters 12,  15 

needed  at  Altona 147 

in  the  United  States 17 

omitted  at  Lawrence..... 97 

Crenothrix 170,  179 

Diarrhoea  due  to  impure  water 144 

Double  filtration  at  Schiedam. 179 

Dresden,  water-supply  of,  from  filter-gallery 182 

Effective  size  of  sand 20,  156 

European  sands 23 

Efficiency  of  filtration 79.  84,  87 

effect  of  rate  upon 46 

effect  of  size  of  sand-grain  upon  25 

effect  of  thickness  of  sand-layer  upon 30 

European  filters 87 

Effluents,  wasting  after  scraping 70,  141 

Elbe,  watershed  of 171 

European  filters,  cost  of 118 

Fseces,  number  of  bacteria  in 133 

Ferric  salts,  use  of 113 

Filling  sand  with  water  from  below 64 

Filter-beds,  bottoms  of,  must  be  water-tight 12,  143 

covers  for 12 

form  of ii 

size  of 10 

Filtered  water,  number  of  people  supplied  with 3 

Filtering  materials. 19 

Filters  at  Altona 172 

at  Berlin 169. 

at  Hamburg 1 7& 

at  London 164 


INDEX.  193 

PACES 

Filters  first  constructed  at  London yg 

for  household  use , ; ug 

general  arrangement  of 6 

regulation  of  rate  of  filtration  of 48,  142 

statistics  of,  at  various  cities    159 

Filtration,  degree  of  purification  required 5 

general  nature  of 5 

FitzGerald,  Desmond.. 69,  104 

Frankel  and  Piefke,  experiments  on  removal  of  disease-germs 82 

Frankfort  on  Main,  water-supply  of,  from  springs 182 

Frankland,  Dr.  Percy 80 

Friction  of  filtered  water  in  pipes 170 

water  in  gravel 33 

water  in  sand 21 

Frictional  resistance  of  underdrains ' 36 

Frost,  effect  of,  at  Altona 13,  147,  172 

effect  of,  upon  filters 12,  147 

Friihling,  on  the  heating  of  water  by  sunshine 16 

underdraining  at  Konigsberg 35 

Fuller,  G.  W . . . 133 

German  Imperial  Board  of  Health 30,  47,  50,  71,  91 

regulations  in  regard  to  filtration 139 

Glasgow,  water-supply  of,  from  Loch  Katrine 181 

Gravel,  layers 31 

friction  of  water  in 33 

screening  of,  for  filters , 33 

Ground- water  supplies 3 

the  use  of,  in  Europe 182 

Halbertsma,  H.  P.  N 50,  55 

Hamburg,  apparatus  for  regulating  depth  of  water  on  filters 55 

health  of ,  , 144 

regulation  of  rate  of  filtration 52 

underdrains  of  filters  at , 38- 

water-supply  of 2,  146,  175 

Hardness,  removal  of 8& 

Havel,  watershed  of  river,  above  Berlin 168 

Household  filters 115 

Ice  on  filters 13 

Impounding  reservoirs 2,  131 

Intermittent  filtration 93 

application  of 104 

at  Chemnitz 100 

at  Lawrence 96 

of  Pegan  Brook 104 

Iron,  metallic,  the  use  of 114 

Kirkwood,  James  P 8,  32,  43,  47>  5L  57,  59,  63 

Koch,  Dr.  Robert 147,  149 

Kraus,  Dr.,  on  the  cause  of  cholera  infantum 140 


IQ4  INDEX. 

PAGES 

Kummel 46,  47,  82 

Lawrence  City  filter,  description  of 96 

Lawrence  Experiment  Station 93 

air  in  water  filtered  in  winter  at 42 

depth  of  sand  removed  at. 66 

depth  of  water  on  filters 42 

effect  of  loss  of  head  upon  efficiency 57 

effect  of  size  of  sand-grain  upon  efficiency 28 

effect  of  size  of  sand-grain  upon  frequency  of  scraping 28 

efficiency  of  filters  at  various  rates 46 

efficiency  of  filtration  at 82,  85 

experiments  with  continuous  filtration , 103 

filters  of  fine  sand 27 

filters  with  various  sand-grain  sizes 28 

gravel  for  filters  at 35 

growth  of  bacteria  in  sterilized  sand  at 81 

intermittent  filtration  investigated 93 

method  of  sand  analysis  at 19,  151 

quantities  of  water  filtered  at  various  losses  of  head 62 

wasting  effluents  not  necessary 71 

Lawrence,  typhoid  fever  at 98,  129 

Lea,  watershed  of  river,  above  London 163 

Leipzig,  water-supply  of,  from  wells , 182 

Lindley 39.  47,  5O,  53,  77 

Literature  of  filtration 183 

Loam  in  filters 31 

London,  water-supply  of 79,  161 

sewage  treatment  on  the  watersheds  of  the  Thames  and  Lea 9,  162,  163 

Loss  of  head 48 

limit  to 56,  63,  142 

reasons  for  allowing  a  high 61 

Magdeburg  filters  at , . .   179 

Manchester,  water-supply  of 181 

Massachusetts  State  Board  of  Health  (see  Lawrence  Experiment  Station). 

Mechanical  filters 106,  137 

reasons  for  the  use  of 108 

Mechanical  filtration  with  alum 109,  137 

Metallic  iron,  use  of  114 

Mills,  H.  F 93,  95,  98 

Miiggel  (Berlin),  filters  at 55,  167 

Munich,  water-supply  of,  from  springs 182 

Nitrification,  effect  of,  upon  bacteria 94 

Objects  of  filtration 122 

Oder,  watershed  of,  above  Breslau 180 

Odors,  removal  of,  by  filtration 122 

Organic  matters  in  water 79 

Paper  manufacturing,  filtration  of  water  for 5,  106 

Paris,  ground-water  supply  of 182 


INDEX.  195 

PAGES 

Passages  through  the  sand  in  filters 63 

Pegan  Brook,  purification  of 104 

Piefke 44,  46,  50,  59,  65,  69,  70,  71,  76,  So,  81,  86 

Plagge  and  Proskower 80 

Plymouth,  Penna.,  typhoid-fever  epidemic  at 132 

Pollution  of  European  water-supplies 89 

Polluted  waters,  utilization  of  excessively 103 

Porcelain  filters  for  household  use 116 

Quantity  of  water  per  capita  in  America. no/ 

Rate  of  filtration 43,  142 

at  various  places 160 

effect  of,  upon  cost 44,  121 

effect  of,  upon  efficiency 46 

lower  after  scraping 72 

regulation  of 48 

Regulation  of  filters 48,  142 

old  forms  of  regulators 48 

modern  forms  of  regulators * 50 

Reincke,  Dr.,  report  on  health  of  Hamburg  for  1892 144 

Renisch  on  the  cause  of  poor  filtration  at  Altona 173 

Reynolds,  Dr.  A.  R. ,  on  Chicago's  water-supply 135 

River  waters,  the  use  of 130 

Roofs  for  filters 16 

Rotterdam,  filters  at 178- 

St.  Louis,  regulators  for  proposed  filters 51 

St.  Petersburg,  filters  at 181 

Samuelson 47 

Sand 19 

analyses  of  European , 23 

analyses  of,  from  leading  works 26 

appliances  for  moving = 64 

compactness  of,  in  natural  banks 57 

depth  of,  in  filters 30,  81 

depth  to  be  removed  from  filters 65 

dune 22 

dune,  washing  of,  impossible 7& 

effect  of  grain  size  upon  the  frequency  of  scraping 28 

effect  of  grain  size  upon  the  efficiency  of  filtration 25 

effective  size  of 20,  156 

extra  scraping  before  replacing  fresh 67 

for  filtration J9.  29 

friction  of  water  in 2I 

grain  size  of J9>  I5I 

in  European  filters 22 

in  Lawrence  filters,  two  sizes  of 9° 

method  of  analysis  of X5i 

quantity  to  be  removed  by  scraping 7<> 

replacing 67 


INDEX. 


PAGBS 

Sand  selection  of  ..................   .........................................     29 

size  of  passages  between  grains  of  ....  .........................  .  ........       6 

thickness  of  layer  ................................................  30,  143,  173 

uniformity  coefficient  ...................................................     20 

Sand  washing  .............................................................  19,  72 

cost  of  .....................................................  „  ,     77 

drum  washers  ..................................................     74 

ejector  washers  ...............  ,  ................................     75 

Greenway's  machine  ............................................     75 

hose  washing  ----  .  ........  ......  ................................     73 

Pegg's  machine  .......  .  ........  ................................     74 

water  for  ...........  .  ....................................  •  ......     76 

Sandstone  filters  for  household  use  ...........................................   116 

Schiedam,  double  filtration  at  ....................  ,  ...........................   179 

Scraping  filters  .............................................  .  ...............       7 

amount  of  labor  required  for  .......  ............................     77 

frequency  of  ................................................  45,  68 

frequency  of,  at  various  places  .................................  160 

Sedgwick,  W.  T  ............................................................     82 

Sediment,  removal  of  .......................................................     88 

Sediment  layer  .............................................................  6,  27 

influence  of  on  bacterial  purification  .............................     80 

thickness  of  ...................  ..............................  29,  62 

Sedimentation  basins  ..........  „  .....................................  8,  164,  175 

effect  of  .............  ........................................   137 

Sewage  treatment  above  London  .......................................  9,  162,  163 

Simpson,  James  ................   ....................  ......................     79 

Spree,  watershed  of,  above  Berlin  ............................  ................   167 

Statistics  of  some  filters  .....................................................   159 

Storage,  effect  of,  upon  quality  of  water  ......................................  136 

Storage  reservoirs,  water  from  ..............................................  2,  131 

Stralau  (Berlin),  filters  at  .................................................  49,  167 

Surface-water,  the  use  of  unfiltered,  in  Europe  .............    ...................  181 

Tastes,  removal  of  ..........    ......  .  ................................  .  .......   122 

Tegel  (Berlin),  filters  at  .................  .  .................................  51,  167 

Thames,  watershed  of,  above  London  ...........  .  .............................   161 

Theory  of  continuous  filtration  ................................................     79 

Tokio,  regulation  of  rate  at  ................................  .  ................     54 

Trenched  bottoms  for  filters  .............................................  32,  36,  96 

Turbidity,  removal  of  .....................................................  88,  122 

Typhoid  fever,  at  Berlin  and  Altona  ...............................  12,  81,  135,  172 

carried  by  personal  contact  ....................................    135 

from  the  use  of  water  filtered  mechanically  .....................   108 

in  American  cities  ...........................................   126 

in  Chicago  ..................................................   135 

in  Lawrence  ...........................  .  .....................     98 

in  London  ................  .  .................................   165 

loss  from..  ................................  •.  .........   124 


INDEX.  IQ7 

PAGES 

Typhoid  fever,  prevention  of 123 

reduction  of,  by  filtration  at  Zurich 181 

Typhoid-fever  germs,  life  of,  in  water 134 

Underdrains 31,  35 

friction  of,  in  Lawrence  filter. 96 

size  of 37 

ventilators  for 40 

Uniformity  coefficient  of  sand .20.  156 

Ventilators  for  underdrains 40 

Vienna,  water-supply  of,  from  springs 182 

Warsaw,  filters  at., 181 

friction  in  underdrains 39 

regulation  of  rate  at 53 

Wasting  effluents 70,  141 

Water,  depth  of,  upon  filters 41 

heating  of,  in  filters 41 

organic  matters  in 79 

Water-supplies  of  American  cities 126 

Water-supply  and  disease 133 

Waters,  what  require  filtration 130 

Winter,  effect  of,  upon  filtration 12,  146,  172 

intermittent  filtration  in 102,  104 

temperatures  at  places  having]open  and  covered  filters 15 

Zurich,  filters  at , ...jj^.u.. 180 


vn  13195 


UNIVERSITY  OF  CAUFORNIA  LIBRARY 


